signer: change the stdio jsonrpc to use legacy namespace conventions (#19047)

This PR will will break existing UIs, since it changes all calls like ApproveSignTransaction to be on the form ui_approveSignTransaction.

This is to make it possible for the UI to reuse the json-rpc library from go-ethereum, which uses this convention.

Also, this PR removes some unused structs, after import/export were removed from the external api (so no longer needs internal methods for approval)

One more breaking change is introduced, removing passwords from the ApproveSignTxResponse and the likes. This makes the manual interface more like the rulebased interface, and integrates nicely with the credential storage. Thus, the way it worked before, it would be tempting for the UI to implement 'remember password' functionality. The way it is now, it will be easy instead to tell clef to store passwords and use them.

If a pw is not found in the credential store, the user is prompted to provide the password.