RFR: 8365400: enhance JFR to emit file and module metadata for class loading

[Larry Cable]

the existing` jdk.classDefine` and `jdk.ClassLoad` events do not include metadata regarding the location/source of the ClassFile.

The location of the class file for a class defined can be of utility in both debugging and auditing.

this addition introduces a new `jdk.ClassFileDefine` event which records the class defined and the location (path/url) of the class file that contained the implementation thereof

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Commit messages:
 - JDK-8365400: changed field label, and normalized value of 'source' when 'null' to 'jvm://'
 - JDK-8365400: added jdk.ClassFileDefine JFR event to record class file source

Changes: https://git.openjdk.org/jdk/pull/27738/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27738&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365400
  Stats: 30 lines in 4 files changed: 27 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27738.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27738/head:pull/27738

PR: https://git.openjdk.org/jdk/pull/27738