gRPC: add support for selected read methods

[kodemartin]

Description

Alongside #7924, the indexer would need a few read methods supported from the gRPC:

• containsTransaction(digest) -> bool that returns whether the transaction has been indexed on the node. Or alternatively getTransaction(digest) -> TransactionBlock by digest, return the transaction block.
• getLatestCheckpointSequenceNumber() -> SequenceNumber

[kodemartin]

@muXxer

[muXxer]

We aligned our implementation with upstream this sprint, so we can reuse the automatic code generation and parts of the indexing needed on the server side. Unfortunately that postponed the implementation of the CheckpointStream. We are on it now.

The getLatestCheckpointSequenceNumber is available in the GetServiceInfo endpoint as executed_checkpoint_height.
The containsTransaction should be done via a GetTransaction request, as you mentioned. The PR for that is currently a draft an will be rebased soon #9335 .