[Request] Credential test endpoint

[tthvo]

Describe the feature

Why don't we just have a connection checking endpoint that specifically does not load any stored credentials? It either uses only the session credentials header for the test, or it ignores the session credentials header and requires the credentials to test to be passed as part of the request body. We can use this endpoint and pass no credentials at first, and just try to open a JMX connection to read some basic MBean data. If that succeeds then we know the target did not require auth. Then the user can supply credentials and we can repeat the test using those.

Originally posted by @andrewazores in cryostatio/cryostat-web#920 (comment)

Any other information?

This is useful for the credential test table.