pageserver/page_api: add binary Protobuf descriptor (#11968)

## Problem

A binary Protobuf schema descriptor can be used to expose an API
reflection service, which in turn allows convenient usage of e.g.
`grpcurl` against the gRPC server.

Touches #11728.

## Summary of changes

* Generate a binary schema descriptor as
`pageserver_page_api::proto::FILE_DESCRIPTOR_SET`.
* Opportunistically rename the Protobuf package from `page_service` to
`page_api`.

pageserver/page_api/src/lib.rs

@@ -7,7 +7,12 @@
 
 // Code generated by protobuf.
 pub mod proto {
-    tonic::include_proto!("page_service");
+    tonic::include_proto!("page_api");
+
+    /// File descriptor set for Protobuf schema reflection. This allows using
+    /// e.g. grpcurl with the API.
+    pub const FILE_DESCRIPTOR_SET: &[u8] =
+        tonic::include_file_descriptor_set!("page_api_descriptor");
 
     pub use page_service_client::PageServiceClient;
     pub use page_service_server::{PageService, PageServiceServer};