deprecate `password_prompt` kwarg to `OP()`

[zcutlip]

We need to deprecate and plan to remove the password_prompt kwarg for the OP() constructor.

Originally the purpose of this kwarg was:

• if the caller didn't provide a password
• and they didn't have a valid OP_SESSION_<token> set from previously authenticating,
• should we allow the op command to prompt for authentication on the console, or just fail hard?

Now that authentications paths have gotten more complex (bio/no bio, service accounts, etc), and we have the existing_auth= kwarg, allowing the caller to specify "use if available", "ignore", or "required", the password_prompt preference doesn't make any sense.

We should do the following:

• refactor _new_or_existing_signin() and _do_normal_signin() where it gets used to rely exclusively on existing_auth
• deprecate the kwarg in OP()
• if the password_prompt is passed in as True, either adjust existing_auth or possibly raise an exception if the values conflict