PR #11956 implemented a new mTLS RPC check to validate the role of the
certificate used in the request, but further testing revealed two flaws:
1. client-only endpoints did not accept server certificates so the
request would fail when forwarded from one server to another.
2. the certificate was being checked after the request was forwarded,
so the check would happen over the server certificate, not the
actual source.
This commit checks for the desired mTLS level, where the client level
accepts both, a server or a client certificate. It also validates the
cercertificate before the request is forwarded.
The Plan.Submit endpoint assumed PlanRequest.Plan was never nil. While
there is no evidence it ever has been nil, we should not panic if a nil
plan is ever submitted because that would crash the leader.
