pack_as_bytes in Authz buffer and Protobuf decoder in Agent

[tanveergill]

Describe the solution you'd like

• Set pack_as_bytes field in Authz buffer settings
• Convert body to UTF-8 before passing to Rego
• We have to make changes on the Agent side to take proto descriptors that can decode protobuf. Use same architecture as OPA server plugin

Additional context

• Not all binary can be represented as UTF-8
• Need binary in order to support protocol parsers in the future that work off the original binary format. E.g. parse query types from database requests

[krdln]

This is blocked on #479