## Problem
The page API gRPC errors need a few tweaks to integrate better with the
GetPage machinery.
Touches https://github.com/neondatabase/neon/issues/11728.
## Summary of changes
* Add `GetPageStatus::InternalError` for internal server errors.
* Rename `GetPageStatus::Invalid` to `InvalidRequest` for clarity.
* Rename `status` and `GetPageStatus` to `status_code` and
`GetPageStatusCode`.
* Add an `Into<tonic::Status>` implementation for `ProtocolError`.
## Problem
For the gRPC Pageserver API, we should convert the Protobuf types to
stricter, canonical Rust types.
Touches https://github.com/neondatabase/neon/issues/11728.
## Summary of changes
Adds Rust domain types that mirror the Protobuf types, with conversion
and validation.
## 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`.
## Problem
For the [communicator
project](https://github.com/neondatabase/company_projects/issues/352),
we want to move to gRPC for the page service protocol.
Touches #11728.
## Summary of changes
This patch adds an experimental gRPC Protobuf schema for the page
service. It is equivalent to the current page service, but with several
improvements, e.g.:
* Connection multiplexing.
* Reduced head-of-line blocking.
* Client-side batching.
* Explicit tenant shard routing.
* GetPage request classification (normal vs. prefetch).
* Explicit rate limiting ("slow down" response status).
The API is exposed as a new `pageserver/page_api` package. This is
separate from the `pageserver_api` package to reduce the dependency
footprint for the communicator. The longer-term plan is to also split
out e.g. the WAL ingestion service to a separate gRPC package, e.g.
`pageserver/wal_api`.
Subsequent PRs will: add Rust domain types for the Protobuf types,
expose a gRPC server, and implement the page service.
Preliminary prototype benchmarks of this gRPC API is within 10% of
baseline libpq performance. We'll do further benchmarking and
optimization as the implementation lands in `main` and is deployed to
staging.
