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.

pageserver/page_api/src/model.rs

@@ -10,6 +10,8 @@
 //!
 //! - Validate protocol invariants, via try_from() and try_into().
 
+use std::fmt::Display;
+
 use bytes::Bytes;
 use postgres_ffi::Oid;
 use smallvec::SmallVec;
@@ -48,7 +50,8 @@ pub struct ReadLsn {
     pub request_lsn: Lsn,
     /// If given, the caller guarantees that the page has not been modified since this LSN. Must be
     /// smaller than or equal to request_lsn. This allows the Pageserver to serve an old page
-    /// without waiting for the request LSN to arrive. Valid for all request types.
+    /// without waiting for the request LSN to arrive. If not given, the request will read at the
+    /// request_lsn and wait for it to arrive if necessary. Valid for all request types.
     ///
     /// It is undefined behaviour to make a request such that the page was, in fact, modified
     /// between request_lsn and not_modified_since_lsn. The Pageserver might detect it and return an
@@ -58,6 +61,17 @@ pub struct ReadLsn {
     pub not_modified_since_lsn: Option<Lsn>,
 }
 
+impl Display for ReadLsn {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let req_lsn = self.request_lsn;
+        if let Some(mod_lsn) = self.not_modified_since_lsn {
+            write!(f, "{req_lsn}>={mod_lsn}")
+        } else {
+            req_lsn.fmt(f)
+        }
+    }
+}
+
 impl ReadLsn {
     /// Validates the ReadLsn.
     pub fn validate(&self) -> Result<(), ProtocolError> {
@@ -584,6 +598,7 @@ impl TryFrom<GetSlruSegmentResponse> for proto::GetSlruSegmentResponse {
     type Error = ProtocolError;
 
     fn try_from(segment: GetSlruSegmentResponse) -> Result<Self, Self::Error> {
+        // TODO: can a segment legitimately be empty?
         if segment.is_empty() {
             return Err(ProtocolError::Missing("segment"));
         }