client: Replace `unsafe_rpc_expose` with an `RpcMethods` enum

which can be either Default, Safe or Unsafe. The idea is to have the
following:
|                       | --rpc-external=false  | --rpc-external=true   |
|---------------------  |-------------------    |-----------------      |
| --rpc-methods=Default |                       | unsafe calls denied   |
| --rpc-methods=Safe    | unsafe calls denied   | unsafe calls denied   |
| --rpc-methods=Unsafe  |                       |                       |
Since the previous `unsafe-rpc-expose` option was confusing.