pageserver/client_grpc: add rich Pageserver gRPC client (#12462)

## Problem

For the communicator, we need a rich Pageserver gRPC client.

Touches #11735.
Requires #12434.

## Summary of changes

This patch adds an initial rich Pageserver gRPC client. It supports:

* Sharded tenants across multiple Pageservers.
* Pooling of connections, clients, and streams for efficient resource
use.
* Concurrent use by many callers.
* Internal handling of GetPage bidirectional streams, with pipelining
and error handling.
* Automatic retries.
* Observability.

The client is still under development. In particular, it needs GetPage
batch splitting, shard map updates, and performance optimization. This
will be addressed in follow-up PRs.

libs/compute_api/src/spec.rs

@@ -442,7 +442,7 @@ pub struct JwksSettings {
 }
 
 /// Protocol used to connect to a Pageserver. Parsed from the connstring scheme.
-#[derive(Clone, Copy, Debug, Default)]
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
 pub enum PageserverProtocol {
     /// The original protocol based on libpq and COPY. Uses postgresql:// or postgres:// scheme.
     #[default]