Don't initialize the signer in solido unless we need to

[ruuda]

Currently we always initialize the signer, even if we are not going to sign anything. Especially when using a Ledger this can be confusing, because it confirms the signer address, so you get Ledger activity even for read-only operations.

Aside from that, it should be possible to run the read-only commands without having a key at all.

Maybe we can add the signer to the client lazily?