JsonRpcRequest: support params as maps (JSON-RPC 2.0 feature)

[msgilligan]

JSON-RPC 2.0 allows the request params value to be a map (JSON object.) Supporting this option has not been critical to any of the existing use-cases for consensusj-jsonrpc but is something that should eventually be supported.

The first step in implementation might be to define a JsonRpcParams type and re-define JsonRpcRequest to use it rather than List<Object> or perhaps even define JsonRpcRequest<T extends JsonRpcParams>.