fix(compute) Correct the PGXN s3 gateway URL. (#11796)

Corrects the postgres extension s3 gateway address to
be not just a domain name but a full base URL.

To make the code more readable, the option is renamed
to "remote_ext_base_url", while keeping the old name
also accessible by providing a clap argument alias.

Also provides a very simple and, perhaps, even redundant
unit test to confirm the logic behind parsing of the
corresponding CLI argument.

## Problem

As it is clearly stated in
https://github.com/neondatabase/cloud/issues/26005, using of the short
version of the domain name might work for now, but in the future, we
should get rid of using the `default` namespace and this is where it
will, most likely, break down.

## Summary of changes

The changes adjust the domain name of the extension s3 gateway to use
the proper base url format instead of the just domain name assuming the
"default" namespace and add a new CLI argument name for to reflect the
change and the expectance.

control_plane/src/bin/neon_local.rs

@@ -644,9 +644,10 @@ struct EndpointStartCmdArgs {
 
     #[clap(
         long,
-        help = "Configure the remote extensions storage proxy gateway to request for extensions."
+        help = "Configure the remote extensions storage proxy gateway URL to request for extensions.",
+        alias = "remote-ext-config"
     )]
-    remote_ext_config: Option<String>,
+    remote_ext_base_url: Option<String>,
 
     #[clap(
         long,
@@ -1414,7 +1415,7 @@ async fn handle_endpoint(subcmd: &EndpointCmd, env: &local_env::LocalEnv) -> Res
         EndpointCmd::Start(args) => {
             let endpoint_id = &args.endpoint_id;
             let pageserver_id = args.endpoint_pageserver_id;
-            let remote_ext_config = &args.remote_ext_config;
+            let remote_ext_base_url = &args.remote_ext_base_url;
 
             let safekeepers_generation = args.safekeepers_generation.map(SafekeeperGeneration::new);
             // If --safekeepers argument is given, use only the listed
@@ -1510,7 +1511,7 @@ async fn handle_endpoint(subcmd: &EndpointCmd, env: &local_env::LocalEnv) -> Res
                     safekeepers_generation,
                     safekeepers,
                     pageservers,
-                    remote_ext_config.as_ref(),
+                    remote_ext_base_url.as_ref(),
                     stripe_size.0 as usize,
                     args.create_test_user,
                     args.start_timeout,

control_plane/src/endpoint.rs

@@ -655,7 +655,7 @@ impl Endpoint {
         safekeepers_generation: Option<SafekeeperGeneration>,
         safekeepers: Vec<NodeId>,
         pageservers: Vec<(Host, u16)>,
-        remote_ext_config: Option<&String>,
+        remote_ext_base_url: Option<&String>,
         shard_stripe_size: usize,
         create_test_user: bool,
         start_timeout: Duration,
@@ -825,8 +825,8 @@ impl Endpoint {
         .stderr(logfile.try_clone()?)
         .stdout(logfile);
 
-        if let Some(remote_ext_config) = remote_ext_config {
-            cmd.args(["--remote-ext-config", remote_ext_config]);
+        if let Some(remote_ext_base_url) = remote_ext_base_url {
+            cmd.args(["--remote-ext-base-url", remote_ext_base_url]);
         }
 
         let child = cmd.spawn()?;