fix(jmcagent): Move jfrprobes_schema.xsd to correct location, add ProbeValidatorTest (backport #452)

[mergify]

Addresses #450

Moves the JFR probes schema to the correct package and adds a ProbeValidator test to make sure this gets caught in future.

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This is an automatic backport of pull request #452 done by Mergify.

[mergify]

Cherry-pick of 6228d49 has failed:

On branch mergify/bp/cryostat-v3.0/pr-452
Your branch is up to date with 'origin/cryostat-v3.0'.

You are currently cherry-picking commit 6228d49.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   cryostat-core/src/test/java/io/cryostat/core/jmcagent/ProbeValidatorTest.java

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both deleted:    cryostat-core/src/main/resources/io/cryostat/core/agent/jfrprobes_schema.xsd
	added by them:   cryostat-core/src/main/resources/io/cryostat/core/jmcagent/jfrprobes_schema.xsd
	added by us:     src/main/resources/io/cryostat/core/agent/jfrprobes_schema.xsd

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