Support `extend` for `caver.rpc`

[jimni1222]

Is your request related to a problem? Please describe.
caver-js should support a way to extend rpc call like extend in web3.

Describe the solution you'd like
caver.rpc.klay.extend
caver.rpc.extend

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
https://web3js.readthedocs.io/en/v1.2.11/web3-eth.html#extend