Implement authentication with application credentials

[gtema]

All necessary provider support is already there.

To be done:

• extend application_credential provider with the "authenticate_by_application_credential" method that should return AuthenticationResult populated with all corresponding details (see authenticate_by_password in identity provider and authorize_by_token from token provider).
• extend crates/api-types/src/v3/auth/token with the new structures for application credential (https://docs.openstack.org/api-ref/identity/v3/index.html#authenticating-with-an-application-credential)
• extend the auth/tokens http handler to determine the application credential in the payload and switch into the corresponding provider method
• add integration tests for authentication by AppCred. Positive and negative tests must be present. Login for disabled user, disabled project, disabled domain must be present and validate login fails
• add api tests with AppCred