feat(v1beta): make FetchBlock, ReadUtxos and ReadData single-item operations

[carbolymer]

Summary

• Remove repeated from FetchBlockRequest.ref and FetchBlockResponse.block, making FetchBlock a single-block operation.
• Remove repeated from ReadUtxosRequest.keys and ReadUtxosResponse.items (now key and item), making ReadUtxos a single-UTxO operation.
• Remove repeated from ReadDataRequest.keys and ReadDataResponse.values (now key and value), making ReadData a single-datum operation.

Motivation

Batching at the protobuf message level adds complexity without clear benefit - callers needing multiple items can issue concurrent requests, which maps naturally to gRPC streaming for bulk use cases.

These RPCs are per-item lookups, but the batched responses had no per-item error channel.
When one item could not be resolved, a server could only fail the whole call or silently omit the item, and the client could not tell omission from an error.
With single-item requests, the gRPC status code applies unambiguously to the one requested item.

This follows the same approach as #163, which made SubmitTx and EvalTx singular.

Breaking change

This is a wire format breaking change to the v1beta proto definition.
A client sending multiple entries in a single request will have message-typed fields silently merged (FetchBlock, ReadUtxos) or resolved as last-value-wins for the bytes key (ReadData), producing incorrect results.
The renamed fields (keys -> key, items -> item, values -> value) also change the JSON mapping.