RFR: 8287: Fix the JMX protocol extenders

[Alex Macdonald]

This PR addresses [JMC-8287](https://bugs.openjdk.org/browse/JMC-8287), in which the JMX protocol extenders were not running due to rjmx.ext not being visible by rjmx.common.

This bug would have originated from the movement of rjmx code from application to core last year.

Making rjmx.ext specify Fragment-Host on rjmx.common fixes the visiblity issue, and the protocol extenders both run as expected. This also fixes use-cases where plugins were expecting to have their protocols used but were failing due to the protocols not being picked up properly:

org.openjdk.jmc.rjmx.common.ConnectionException caused by java.net.MalformedURLException: Unsupported protocol:

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Commit messages:
 - 8287: Fix the JMX protocol extenders

Changes: https://git.openjdk.org/jmc/pull/602/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=602&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8287
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jmc/pull/602.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/602/head:pull/602

PR: https://git.openjdk.org/jmc/pull/602