RFR: 6757: Selector for attribute in the flame graph

[Jean-Philippe Bempel]

Add support for aggregating flamegraph by custom attribute.

Available attributes are chosen by their content type (number or memory).

<img width="1435" alt="Screenshot 2022-04-22 at 08 35 
47" src="https://user-images.githubusercontent.com/4610701/164617514-34d56474-768a-4f96-8d92-5ba617aca45a.png">

<img width="1437" alt="Screenshot 2022-04-22 at 08 36 06" src="https://user-images.githubusercontent.com/4610701/164617547-c0da25a8-d5f2-4739-bbd9-d41a3dd98d02.png">

Added support on StacktraceView:

<img width="1098" alt="Screenshot 2022-05-10 at 22 20 13" src="https://user-images.githubusercontent.com/4610701/167715364-04ca6580-4389-4c21-b6da-5d7d0fcae63b.png">

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Commit messages:
 - 6757: Selector for attribute in the flame graph

Changes: https://git.openjdk.java.net/jmc/pull/365/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=365&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6757
  Stats: 376 lines in 11 files changed: 324 ins; 0 del; 52 mod
  Patch: https://git.openjdk.java.net/jmc/pull/365.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/365/head:pull/365

PR: https://git.openjdk.java.net/jmc/pull/365