New algod endpoint: /v2/block/{round}/hash

[winder]

A new endpoint to fetch the block hash for a given round. The block endpoint provides a direct representation of the raw block, and doesn't provide the hash computed from the raw data. To avoid complicating that endpoint we should make a new endpoint at /v2/block/{round}/hash which returns

{
    "hash": ...
}

This is a small feature with many downstream dependencies:

• [ ] add new endpoint to go-algorand
• [ ] add new unit test to algorand-sdk-testing
• [ ] go SDK
• [ ] js SDK
• [ ] java SDK
• [ ] python SDK