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.

pageserver/src/page_service.rs

@@ -3353,6 +3353,8 @@ impl GrpcPageServiceHandler {
     /// NB: errors returned from here are intercepted in get_pages(), and may be converted to a
     /// GetPageResponse with an appropriate status code to avoid terminating the stream.
     ///
+    /// TODO: verify that the requested pages belong to this shard.
+    ///
     /// TODO: get_vectored() currently enforces a batch limit of 32. Postgres will typically send
     /// batches up to effective_io_concurrency = 100. Either we have to accept large batches, or
     /// split them up in the client or server.