MB-61292: Add support for secrets and key deletion

1. Implementation for DELETE /settings/secrets/<id>
2. Garbage collection for keks

Change-Id: I9baa6cf45a8e626fc67cdca4fb953986e99d74cd
Reviewed-on: https://review.couchbase.org/c/ns_server/+/211957
Tested-by: Timofey Barmin <[email protected]>
Reviewed-by: Navdeep S Boparai <[email protected]>
Well-Formed: Build Bot <[email protected]>

Author: timofey-barmin