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/utils/src/shard.rs

@@ -171,6 +171,12 @@ impl std::fmt::Display for ShardNumber {
     }
 }
 
+impl std::fmt::Display for ShardCount {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
 impl std::fmt::Display for ShardSlug<'_> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(