feat(triggers): Add support for querying mbean attributes

[Josh-Matsuoka]

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Fixes: #

Depends on: cryostatio/cryostat-core#667 cryostatio/cryostat-agent#849

Description of the change:

• Adds a new endpoint /targets/{targetId}/mbean-query that returns all currently available mbean attributes. Required for the smart triggers mbean selector form, and in future can be used to build an mbean viewer page in the frontend or other similar components.

Motivation for the change:

This change is helpful because users may want to...

How to manually test:

1. Run CRYOSTAT_IMAGE=quay.io... bash smoketest.bash...
2. ...