Allow unlink oauth_account from user

[gromdimon]

Cannot unlink oauth_account from user

Describe the bug
Currently we can only link new OAuth methods to existing users. However, it'll be useful to have a possibility to unlink this methods.

Expected behavior

1. There should be some similar route, which will allow to unlink the OAuth method from user.

# Add route for associating OAuth2 account with existing user.
oauth_associate_router = fastapi_users.get_oauth_associate_router(
    oauth_client=oauth_client,
    user_schema=UserRead,
    state_secret=settings.SECRET_KEY,
    redirect_url="/",
)
api_router.include_router(
    oauth_associate_router,
    prefix=f"/auth/external/associate/cookie/{config.name}",
    tags=["auth"],
)

The corresponding question was asked in fastapi-users discusssion

2. The functionality to unlink OAuth account should be presented in Profile

Additional context
FastAPI-users discussions:
fastapi-users/fastapi-users#1359
fastapi-users/fastapi-users#1360