RFR: 7992: Add API to easily write annotated Java JFR events [v2]

[Jaroslav Bachorik]

> This PR extends the JFR Writer API with the ability to use the custom JFR event types (eg. classes extending `jdk.jfr.Event`) and register new writer type for them and also directly accept the instances of those types to write them in the recording.

Jaroslav Bachorik has updated the pull request incrementally with two additional commits since the last revision:

 - Add more explanatory comments
 - Add javadoc for the new public methods

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Changes:
  - all: https://git.openjdk.org/jmc/pull/457/files
  - new: https://git.openjdk.org/jmc/pull/457/files/eecdda83..e4e6c7cf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jmc&pr=457&range=01
 - incr: https://webrevs.openjdk.org/?repo=jmc&pr=457&range=00-01

  Stats: 19 lines in 2 files changed: 19 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jmc/pull/457.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/457/head:pull/457

PR: https://git.openjdk.org/jmc/pull/457