RFR: 6715: Create MXBean retrieveCurrentTransforms function

[Jessye Coleman-Shapiro]

This patch adds the MXBean function retrieveCurrentTransforms to the agent.  This allows a user to get information
about the transforms that are currently instrumented.  I have added a test "testRetrieveCurrentTransforms".

Let me know what you think!

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Commit messages:
 - Create MXBean retrieveCurrentTransforms function

Changes: https://git.openjdk.java.net/jmc/pull/68/files
 Webrev: https://webrevs.openjdk.java.net/jmc/68/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6715
  Stats: 310 lines in 14 files changed: 265 ins; 37 del; 8 mod
  Patch: https://git.openjdk.java.net/jmc/pull/68.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/68/head:pull/68

PR: https://git.openjdk.java.net/jmc/pull/68