Move secret redaction logic to elastic-agent-libs

Elastic Agent and Fleet Server uses a common secret redaction logic that should be moved to a common place:

https://github.com/elastic/elastic-agent/blob/1c3fb4b4c8989cd2cfb692780debd7619820ae72/internal/pkg/diagnostics/diagnostics.go#L454-L468

https://github.com/elastic/fleet-server/pull/6400/changes#diff-8eae4f76576728f1316ae744b620c2dac1f0ea3964426990df7329dcb9fff745R443