Copy pg server cert and key to pgdata with correct permission

## Problem

We need to 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.

Co-authored-by: Tristan Partin <tristan.partin@databricks.com>

compute_tools/src/compute.rs

@@ -1462,6 +1462,14 @@ impl ComputeNode {
         // Update pg_hba.conf received with basebackup.
         update_pg_hba(pgdata_path, None)?;
 
+        if let Some(databricks_settings) = spec.databricks_settings.as_ref() {
+            copy_tls_certificates(
+                &databricks_settings.pg_compute_tls_settings.key_file,
+                &databricks_settings.pg_compute_tls_settings.cert_file,
+                pgdata_path,
+            )?;
+        }
+
         // Place pg_dynshmem under /dev/shm. This allows us to use
         // 'dynamic_shared_memory_type = mmap' so that the files are placed in
         // /dev/shm, similar to how 'dynamic_shared_memory_type = posix' works.

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

libs/compute_api/src/spec.rs

@@ -193,6 +193,10 @@ pub struct ComputeSpec {
     ///
     /// We use this value to derive other values, such as the installed extensions metric.
     pub suspend_timeout_seconds: i64,
+
+    // Databricks specific options for compute instance.
+    // These settings are not part of postgresql.conf.
+    pub databricks_settings: Option<DatabricksSettings>,
 }
 
 /// Feature flag to signal `compute_ctl` to enable certain experimental functionality.