RFR: 6905: Add rule for heap dumps

[Jean-Philippe Bempel]

Rule based on ne JDK15 jdk.HeapDump event.
If at least on heap dump is performed, rule indicates it as information, score 50.
For test added a new JFR file containing an example where 2 dumps were performed

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Commit messages:
 - Add rule for heap dumps

Changes: https://git.openjdk.java.net/jmc/pull/113/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=113&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6905
  Stats: 571 lines in 8 files changed: 571 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/113.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/113/head:pull/113

PR: https://git.openjdk.java.net/jmc/pull/113