rpc: Add options for limiting getConfirmedBlock response data

[jstarry]

Problem

The getConfirmedBlock RPC response can be quite massive when considering transaction details and metadata as well as a ton of reward information for blocks on the epoch boundary. Not all consumers will need this information but currently everyone has to pay the cost of bandwidth and response time.

Proposed Solution

1. Add an option to not return reward info
2. Add an option to return either full tx details, only signatures, or nothing at all.

[mvines]

The "nothing at all" case for transactions in a block would speed up the rewards output in solana vote-account and solana stake-account -- these two commands are getting slower and slower for each new epoch post-inflation. This is probably worth spinning off into another issue, once this issue is fixed, assuming it doesn't happen at the same time