Copy pg server cert and key to pgdata with correct permission (#12731)

## Problem

Copy certificate and key from secret mount directory to `pgdata`
directory where `postgres` is the owner and we can set the key
permission to 0600.

## Summary of changes

- Added new pgparam `pg_compute_tls_settings` to specify where k8s
secret for certificate and key are mounted.
- Added a new field to `ComputeSpec` called `databricks_settings`. This
is a struct that will be used to store any other settings that needs to
be propagate to Compute but should not be persisted to `ComputeSpec` in
the database.
- Then when the compute container start up, as part of `prepare_pgdata`
function, it will copied `server.key` and `server.crt` from k8s mounted
directory to `pgdata` directory.

## How is this tested?

Add unit tests.
Manual test via KIND

Co-authored-by: Jarupat Jisarojito <jarupat.jisarojito@databricks.com>

control_plane/src/endpoint.rs

@@ -793,6 +793,7 @@ impl Endpoint {
                 autoprewarm: args.autoprewarm,
                 offload_lfc_interval_seconds: args.offload_lfc_interval_seconds,
                 suspend_timeout_seconds: -1, // Only used in neon_local.
+                databricks_settings: None,
             };
 
             // this strange code is needed to support respec() in tests