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.

libs/pageserver_api/src/models.rs

@@ -2045,7 +2045,7 @@ pub enum PagestreamProtocolVersion {
 
 pub type RequestId = u64;
 
-#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+#[derive(Debug, Default, PartialEq, Eq, Clone, Copy)]
 pub struct PagestreamRequest {
     pub reqid: RequestId,
     pub request_lsn: Lsn,
@@ -2064,7 +2064,7 @@ pub struct PagestreamNblocksRequest {
     pub rel: RelTag,
 }
 
-#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+#[derive(Debug, Default, PartialEq, Eq, Clone, Copy)]
 pub struct PagestreamGetPageRequest {
     pub hdr: PagestreamRequest,
     pub rel: RelTag,

libs/pageserver_api/src/reltag.rs

@@ -24,7 +24,7 @@ use serde::{Deserialize, Serialize};
 // FIXME: should move 'forknum' as last field to keep this consistent with Postgres.
 // Then we could replace the custom Ord and PartialOrd implementations below with
 // deriving them. This will require changes in walredoproc.c.
-#[derive(Debug, PartialEq, Eq, Hash, Clone, Copy, Serialize, Deserialize)]
+#[derive(Debug, Default, PartialEq, Eq, Hash, Clone, Copy, Serialize, Deserialize)]
 pub struct RelTag {
     pub forknum: u8,
     pub spcnode: Oid,