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/pageserver_api/src/shard.rs

@@ -332,7 +332,11 @@ fn hash_combine(mut a: u32, mut b: u32) -> u32 {
 ///
 /// The mapping of key to shard is not stable across changes to ShardCount: this is intentional
 /// and will be handled at higher levels when shards are split.
-fn key_to_shard_number(count: ShardCount, stripe_size: ShardStripeSize, key: &Key) -> ShardNumber {
+pub fn key_to_shard_number(
+    count: ShardCount,
+    stripe_size: ShardStripeSize,
+    key: &Key,
+) -> ShardNumber {
     // Fast path for un-sharded tenants or broadcast keys
     if count < ShardCount(2) || key_is_shard0(key) {
         return ShardNumber(0);