pagebench: add gRPC support for get-page-latest-lsn (#12077)

## Problem

We need gRPC support in Pagebench to benchmark the new gRPC Pageserver
implementation.

Touches #11728.

## Summary of changes

Adds a `Client` trait to make the client transport swappable, and a gRPC
client via a `--protocol grpc` parameter. This must also specify the
connstring with the gRPC port:

```
pagebench get-page-latest-lsn --protocol grpc --page-service-connstring grpc://localhost:51051
```

The client is implemented using the raw Tonic-generated gRPC client, to
minimize client overhead.

Cargo.lock

@@ -4236,6 +4236,7 @@ name = "pagebench"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "camino",
  "clap",
  "futures",
@@ -4244,12 +4245,15 @@ dependencies = [
  "humantime-serde",
  "pageserver_api",
  "pageserver_client",
+ "pageserver_page_api",
  "rand 0.8.5",
  "reqwest",
  "serde",
  "serde_json",
  "tokio",
+ "tokio-stream",
  "tokio-util",
+ "tonic 0.13.1",
  "tracing",
  "utils",
  "workspace_hack",