Workloadapi FetchX509SVID logs selectors at info on success

[zmt]

Pull Request check list

• Commit conforms to CONTRIBUTING.md?
• Proper tests/regressions included?
• Documentation updated?

Affected functionality
Agent logging on successful FetchX509SVID.

Description of change
Agent logs at info level with selectors on successful FetchX509SVID.

Initial follow-up to discussion in #6179, relates to #2651, and probably depends on #2666 (closed unplanned).

[amartinezfayo]

We discussed this in the SPIRE Contributor call and concluded that logging a “Fetched X.509 SVID” message at the Info level each time an X509-SVID is fetched could overwhelm the agent logs.

This might be a good opportunity to introduce Audit Logging in the agent, similar to what we already have in the server. That way, this event could be captured as an audit log, allowing users to opt in if they want to track fetched X.509 SVIDs.

Even though selectors may change after attestation, I think it’s still valuable to include selectors in the audit log entry. Users should be able to have the selectors redacted.

[zmt]

I'm going to abandon this for now. I like the suggestion of adding audit log capability to spire-agent.