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

[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.

### Dependency management

I've 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 �� I don't understand why this is happening or how to fix it.


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)"

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

Commit messages:
 - Require specific jetty bundle version to avoid conflict in ui tests
 - Remove explicit dependency on javax.servlet-api
 - Refactor WebsocketServer and embed inside JfrEditor
 - Add proof of concept websocket server implementation
 - Add listener in JfrEditor for websocket configuration changes
 - Add dependency on javax.websocket and eclipse jetty in flightrecorder.ui
 - Add setting to enable websocket server in preferences

Changes: https://git.openjdk.java.net/jmc/pull/306/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=306&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7198
  Stats: 379 lines in 18 files changed: 324 ins; 0 del; 55 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