rpc,security: set up TLS parameters for pod-to-pod communication

[knz]

The rpc.Context logic today is equipped to set up TLS parameters for:

• Client-to-KV RPCs
• KV-to-KV RPCs
• SQL pod to KV RPCs

Now we also want to introduce SQL pod to SQL pod RPCs. We need a new TLS configuration for that.

Some questions to answer:

• which CA to use?
• are there other "gotchas" to account for?

Epic: CRDB-8505

[knz]

cc @dhartunian