RFR: 7198: add websocket server that pushes data on selection updates [v9]

[Alex Ciminian]

> This PR adds a websocket server that pushes event data as JSON to all connected clients whenever the user updates their current selection in JMC. The server is disabled by default and can be enabled and disabled from JMC preferences. This is a rework of the proof-of-concept code in https://github.com/openjdk/jmc/pull/225 and uses the JSON serialiser introduced in #279.
> 
> ![Screenshot 2021-09-15 at 12 08 56](https://user-images.githubusercontent.com/348973/133478041-ee436fbd-de3a-471d-955d-d0fe4cc854bf.png)
> 
> ### Dependency management
> 
> 
> Brice pointed out that there are newer versions of the Jetty websocket server available under a different artefact name - I used the same version as required in the UI tests (`10.0.5`) and this made the build succeed in both eclipse and CLI. I've had to add SPI fly since it was required by jetty, but I don't really understand what it does.
> 
> Initially I struggled to add the required dependencies to the project. Initially I was only testing in eclipse with the setup from #225 and it worked fine.`mvn package` also worked on the command line. However, the spotless task from our CI script failed with the following error:
> 
> 
> org.apache.felix.resolver.reason.ReasonException: Uses constraint violation. Unable to resolve resource org.openjdk.jmc.flightrecorder.ui [osgi.identity; osgi.identity="org.openjdk.jmc.flightrecorder.ui"; type="osgi.bundle"; version:Version="8.2.0.qualifier"; singleton:="true"] because it is exposed to package 'javax.servlet' from resources javax.servlet [osgi.identity; osgi.identity="javax.servlet"; type="osgi.bundle"; version:Version="3.1.0.v201410161800"] and jakarta.servlet-api [osgi.identity; osgi.identity="jakarta.servlet-api"; type="osgi.bundle"; version:Version="4.0.0"] via two dependency chains.
> [ERROR] 
> [ERROR] Chain 1:
> [ERROR]   org.openjdk.jmc.flightrecorder.ui [osgi.identity; osgi.identity="org.openjdk.jmc.flightrecorder.ui"; type="osgi.bundle"; version:Version="8.2.0.qualifier"; singleton:="true"]
> [ERROR]     require: (osgi.wiring.bundle=javax.servlet)
> [ERROR]      |
> [ERROR]     provide: osgi.wiring.bundle: javax.servlet
> [ERROR]   javax.servlet [osgi.identity; osgi.identity="javax.servlet"; type="osgi.bundle"; version:Version="3.1.0.v201410161800"]
> [ERROR] 
> [ERROR] Chain 2:
> [ERROR]   org.openjdk.jmc.flightrecorder.ui [osgi.identity; osgi.identity="org.openjdk.jmc.flightrecorder.ui"; type="osgi.bundle"; version:Version="8.2.0.qualifier"; singleton:="true"]
> [ERROR]     require: (osgi.wiring.bundle=org.eclipse.jetty.server)
> [ERROR]      |
> [ERROR]     provide: osgi.wiring.bundle; bundle-version:Version="10.0.5"; osgi.wiring.bundle="org.eclipse.jetty.server"
> [ERROR]   org.eclipse.jetty.server [osgi.identity; osgi.identity="org.eclipse.jetty.server"; type="osgi.bundle"; version:Version="10.0.5"]
> [ERROR]     import: (&(osgi.wiring.package=javax.servlet)(&(version>=4.0.0)(!(version>=5.0.0))))
> [ERROR]      |
> [ERROR]     export: osgi.wiring.package: javax.servlet
> [ERROR]   jakarta.servlet-api [osgi.identity; osgi.identity="jakarta.servlet-api"; type="osgi.bundle"; version:Version="4.0.0"]
> 
> 
> This is because the task is run with the `-P uitests` and something in the UI test configuration pulls in jetty.server 10.0.5. Since [the latest available version](https://mvnrepository.com/artifact/org.eclipse.jetty.websocket/websocket-server) for the jetty websocket server is `9.4.43.v20210629` we can't use the newer `10.x` for the transitive dependencies.
> 
> I was able to work around this problem by setting bundle-version constraints in the `flightrecorder.ui` manifest. This makes all CI checks pass, but unfortunately it also breaks the JFR editor view in eclipse �� 
> 
> 
> org.osgi.framework.BundleException: Could not resolve module: org.openjdk.jmc.joverflow.ui [457]
>   Unresolved requirement: Import-Package: org.openjdk.jmc.flightrecorder.ui
>     -> Export-Package: org.openjdk.jmc.flightrecorder.ui; bundle-symbolic-name="org.openjdk.jmc.flightrecorder.ui"; bundle-version="8.2.0.qualifier"; version="0.0.0"
>        org.openjdk.jmc.flightrecorder.ui [445]
>          Unresolved requirement: Require-Bundle: org.eclipse.jetty.server; bundle-version="[9.4.43,10.0.0)"

Alex Ciminian has updated the pull request incrementally with one additional commit since the last revision:

  Add additional asm and asm.tree plugins to eclipse launchers

-------------

Changes:
  - all: https://git.openjdk.java.net/jmc/pull/306/files
  - new: https://git.openjdk.java.net/jmc/pull/306/files/76fe428e..b5b6adef

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=306&range=08
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=306&range=07-08

  Stats: 12 lines in 3 files changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/306.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/306/head:pull/306

PR: https://git.openjdk.java.net/jmc/pull/306