Addition of Starknet API Version RPC Endpoint

JSON-RPC API is still evolving and backward compatibility is not guaranteed.
There is no way to know which JSON-RPC version is supported by a given node.

With a starnet_apiVersion JSON-RPC endpoint, I think it will help SDK builders and developers to create more reliable applications.

Here is my proposal for the endpoint:

   "name": "starknet_apiVersion",
   "summary": "Get JSON RPC API version",
   "params": [],
   "result": {
       "name": "version",
       "description": "API version used by provider",
       "type": "string"
   "errors": []