RFR: 7167: Agent Plugin [v2]

[Joshua Matsuoka]

> This PR adds the Agent Plugin to JMC. This is a plugin for controlling the jmc agent and provides a way to attach it and add probes to any JVMs in the JVM browser, as well as providing a preset manager for creating/modifying probe presets.
> 
> There is still a small amount of cleanup to be done so I'll mark it as a draft for now but it would be great to get a review :)

Joshua Matsuoka has updated the pull request incrementally with one additional commit since the last revision:

  Removing ext from agent plugin identifier, fixing bundle version

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Changes:
  - all: https://git.openjdk.java.net/jmc/pull/226/files
  - new: https://git.openjdk.java.net/jmc/pull/226/files/789298d7..a87cbb7d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=226&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=226&range=00-01

  Stats: 377 lines in 61 files changed: 115 ins; 115 del; 147 mod
  Patch: https://git.openjdk.java.net/jmc/pull/226.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/226/head:pull/226

PR: https://git.openjdk.java.net/jmc/pull/226