Implement the password DB as a connector

[sagikazarmark]

Is your feature request related to a problem?

Currently the password DB can be enabled and configured separately from the rest of the connectors.

Describe the solution you'd like to see

Let's find a way to make it a regular connector. Since Dex's gRPC API provides an interface for adding new passwords, this connector should not be instantiated more than once.

Additional context

Deprecate the current password DB and consider reimplementing it as an external connector: #1907