Add Block Encoding Param to Block Index API

[aaronbuchwald]

Add an encoding parameter to the block index API requests to specify either a binary (hex) or JSON encoding.

We currently include both to support easy to handle JSON output for a barebones explorer: #1513 and require a binary output so the HyperSDK itself can decode into an ExecutedBlock type.

HyperSDK requires the binary so that it can decode while passing in the action/auth registries to handle the Action and Auth types included in transactions.