Add get_username_and_password API

[zooba]

There are some scenarios where it is possible (and preferred) to retrieve the username for a service from just the system, but currently keyring has no ability to return a username that has not been provided by the caller.

I propose to add a new API matching get_password's signature, but named get_username_and_password ad returning (username, password), where username might match the provided username. The ABC can include a default implementation of this that uses get_password and returns the provided username.

[zooba]

I intend to work on this today at the PyPA sprints.

[zooba]

Thinking about set_username_and_password, there doesn't seem to be any value in supporting this. It's really only useful for "read-only" keyring backends, since setting a password implies setting the username too.