Add block proof to GetBlockByNumber endpoint

[sergerad]

Modify the endpoint to allow users to specify whether they want a block proof to also be returned.

Both the fn argument and return type must be updated for this purpose:

    // Returns raw block data for the specified block number.
    rpc GetBlockByNumber(blockchain.BlockNumber) returns (blockchain.MaybeBlock) {}

Relevant comments

• fix: Genesis(SignedBlock) #1858 (review)
• fix: Genesis(SignedBlock) #1858 (comment)