Validators extension

[volovyks]

Create a simple validators extension that will have the same functionality as the one that is available in [near-cli](https://github.com/near/near-cli).
near validators <epoch> and near proposals commands in particular.
It should be a separate crate inside of the near-cli-rs repo.

[frol]

1. I can recommend using the new https://github.com/near/near-jsonrpc-client-rs/
2. I feel this is somewhat blocked on Extract the interactive/cli interface into a standalone crate #47