RFR: 7167: Agent Plugin [v3]

[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:

  Fixing build issues, cleanup

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

Changes:
  - all: https://git.openjdk.java.net/jmc/pull/226/files
  - new: https://git.openjdk.java.net/jmc/pull/226/files/a87cbb7d..11b1fb15

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=226&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=226&range=01-02

  Stats: 77 lines in 74 files changed: 19 ins; 17 del; 41 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