fix(registration): apply mitigations to Cryostat registration flooding

[andrewazores]

Related to cryostatio/cryostat#1483

This should improve the robustness and self-healing ability of the Agent's registration as a Discovery Plugin.

1. Applies a circuit-breaker and cooldown to registration attempts. Previously the Agent had a simple fixed timer interval that it would re-attempt registrations after a failure. Now, it will slow down attempts on failure until hitting the circuit breaker, at which point it will take a long pause before retrying.
2. The Agent will now refuse requests on its own WebServer if it is in a state where it isn't expecting them - ie if it believes its own registration process has not yet started, or it's in circuit-breaker cooldown. This ensures that if Cryostat tries to ping plugins based on records it has in its database, but the Agent doesn't think it is actually registered, the Agent should not respond to Cryostat. This way Cryostat can mark the plugin as failed and prune it, so that the Agent can hopefully retry again later and succeed.
3. Adds a CredentialTracker and some cleaner shutdown logic to ensure the Agent does a better best-effort job of cleaning up after itself and removing its credentials from Cryostat's keyring.