pageserver: initial gRPC page service implementation (#12094)

## Problem

We should expose the page service over gRPC.

Requires #12093.
Touches #11728.

## Summary of changes

This patch adds an initial page service implementation over gRPC. It
ties in with the existing `PageServerHandler` request logic, to avoid
the implementations drifting apart for the core read path.

This is just a bare-bones functional implementation. Several important
aspects have been omitted, and will be addressed in follow-up PRs:

* Limited observability: minimal tracing, no logging, limited metrics
and timing, etc.
* Rate limiting will currently block.
* No performance optimization.
* No cancellation handling.
* No tests.

I've only done rudimentary testing of this, but Pagebench passes at
least.

libs/pageserver_api/src/models.rs

@@ -1934,7 +1934,7 @@ pub enum PagestreamFeMessage {
 }
 
 // Wrapped in libpq CopyData
-#[derive(strum_macros::EnumProperty)]
+#[derive(Debug, strum_macros::EnumProperty)]
 pub enum PagestreamBeMessage {
     Exists(PagestreamExistsResponse),
     Nblocks(PagestreamNblocksResponse),

libs/pageserver_api/src/reltag.rs

@@ -184,12 +184,12 @@ pub enum SlruKind {
     MultiXactOffsets,
 }
 
-impl SlruKind {
-    pub fn to_str(&self) -> &'static str {
+impl fmt::Display for SlruKind {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
-            Self::Clog => "pg_xact",
-            Self::MultiXactMembers => "pg_multixact/members",
-            Self::MultiXactOffsets => "pg_multixact/offsets",
+            Self::Clog => write!(f, "pg_xact"),
+            Self::MultiXactMembers => write!(f, "pg_multixact/members"),
+            Self::MultiXactOffsets => write!(f, "pg_multixact/offsets"),
         }
     }
 }