pageserver: payload compression for gRPC base backups (#12346)

## Problem

gRPC base backups use gRPC compression. However, this has two problems:

* Base backup caching will cache compressed base backups (making gRPC
compression pointless).
* Tonic does not support varying the compression level, and zstd default
level is 10% slower than gzip fastest level.

Touches https://github.com/neondatabase/neon/issues/11728.
Touches https://github.com/neondatabase/cloud/issues/29353.

## Summary of changes

This patch adds a gRPC parameter `BaseBackupRequest::compression`
specifying the compression algorithm. It also moves compression into
`send_basebackup_tarball` to reduce code duplication.

A follow-up PR will integrate the base backup cache with gRPC.

pageserver/pagebench/src/cmd/basebackup.rs

@@ -317,6 +317,7 @@ impl Client for LibpqClient {
 /// A gRPC Pageserver client.
 struct GrpcClient {
     inner: page_api::Client,
+    compression: page_api::BaseBackupCompression,
 }
 
 impl GrpcClient {
@@ -331,10 +332,14 @@ impl GrpcClient {
             ttid.timeline_id,
             ShardIndex::unsharded(),
             None,
-            compression.then_some(tonic::codec::CompressionEncoding::Zstd),
+            None, // NB: uses payload compression
         )
         .await?;
-        Ok(Self { inner })
+        let compression = match compression {
+            true => page_api::BaseBackupCompression::Gzip,
+            false => page_api::BaseBackupCompression::None,
+        };
+        Ok(Self { inner, compression })
     }
 }
 
@@ -348,6 +353,7 @@ impl Client for GrpcClient {
             lsn,
             replica: false,
             full: false,
+            compression: self.compression,
         };
         let stream = self.inner.get_base_backup(req).await?;
         Ok(Box::pin(StreamReader::new(