Add ETH RPC API to watcher server

[prathamesh0]

Part of Watcher ETH JSON-RPC wrapper

• Added a JSON RPC server at /rpc path to the watcher server behind a flag enableEthRPCServer
• Added support for eth_blockNumber and eth_call methods
  • Currently supported block tags: block hash, block number, latest
• To be handled in a follow on PR:
  • Add support for eth_getLogs method
  • Handle GET requests to match Geth's behaviour
  • Required codegen changes