## Problem
With gRPC `GetPageRequest` batches, we'll have non-trivial
fragmentation/reassembly logic in several places of the stack
(concurrent reads, shard splits, LFC hits, etc). If we included the
block numbers with the pages in `GetPageResponse` we could have better
verification and observability that the final responses are correct.
Touches #11735.
Requires #12480.
## Summary of changes
Add a `Page` struct with`block_number` for `GetPageResponse`, along with
the `RelTag` for completeness, and verify them in the rich gRPC client.
8aa9540a05