Support custom extensions.

Add infrastructure to dynamically load postgres extensions and shared libraries from remote extension storage.

Before postgres start  downloads list of available remote extensions and libraries, and also downloads 'shared_preload_libraries'.
After postgres is running, 'compute_ctl' listens for HTTP requests to load files.

Postgres has new GUC 'extension_server_port' to specify port on which 'compute_ctl' listens for requests.

When PostgreSQL requests a file, 'compute_ctl'  downloads it.

See more details about feature design and remote extension storage layout in docs/rfcs/024-extension-loading.md

libs/compute_api/src/spec.rs

@@ -60,6 +60,9 @@ pub struct ComputeSpec {
     /// If set, 'storage_auth_token' is used as the password to authenticate to
     /// the pageserver and safekeepers.
     pub storage_auth_token: Option<String>,
+
+    // list of prefixes to search for custom extensions in remote extension storage
+    pub custom_extensions: Option<Vec<String>>,
 }
 
 #[serde_as]