Ability to store metadata/settings as part of Client <-> User relationship

[eshaan7]

There can be use cases where developers wish to store settings/metadata about a user specific to a client. (For example, theme name, styling, avatar, display name, etc.)

Few initial implementation ideas:

• Provide an abstract UserClient model with user_id and client_id fields that developers could subclass.
• Provide an UserClientSettings model with user_id, client_id, metadata = JsonField fields that developers can directly use.

Model should provide a shortcut method:

def get_current(request: "rest_framework.request.Request") -> UserClient

More ideas welcome.

[eshaan7]

Some initial code is there in branch: dev-issue7