Framework credential manager developer APIs definitions.

The CredentialManager module provides solution for developers to create
or get credential from a user that can be used to authenticate to the app.
It can potentially launch system level UI flows to collect user consent,
render a unified selector when more than one credential exists, and
perform anything else needed.

Check bug id for detailed design.

Note that this is still very early stage of productionizing the module
and proper test cases will be added subsequently when the service
gets more concrete business logic.

Bug: 246564035
CTS-Coverage-Bug: 246637346
Test: Local Build & Deployment
Change-Id: Ia5884a7f6a3908de3b8b4b29f154f98e25730bd7