Add web GUI support

[gdong42]

For users that develop gRPC services alongside gRPC Mate, a web UI of the latter is handy. User can check with services/methods they are working on, and quickly shoot a gRPC call with JSON payload through the web UI and check the results.

Although one can achieve the same using CLI commands, it would be more efficient to click around using a UI while he/she is debugging.

Features Must Have:

• Show information of the backend gRPC service it connects to, e.g. the address, the port, etc
• List all services and methods the backend exposes.
• Easy to construct a request with selected service/method, with optional input message ( better pre-filled with sample values)
• Show the result status code and result message, or error message if the request is not successful

Features Nice to Have:

• Health check status

Some considerations on implementation:

• it should be opt-in by an optional command line flag, e.g. -ui
• the static resources including the template files should be packaged into the standalone executable binary.