RPC Client: take signing key from node config when creating a validator

[styppo]

Use configured signing key when creating a CreateValidator transaction with the RPC client instead of passing the signing key via RPC.

[sisou]

It should be possible to create such a transaction on another server, too though, no? On one which doesn't have the key configured.

[sisou]

The parameter could be made optional and fall back to any configured key, or return an error if not configured.

[sisou]

I updated the title and description to clarify that this is about the RPC client subcommands, not about the server API.