Manage REST auth parameters on UI

As an Admin I'd like to manage REST authentication parameters (username & password) on UI

Acceptance Criteria:

• Given I open properties of Poulpe component, I should be able to see REST Auth Username & REST Auth Password fields. They should be simple text fields.
• Given I've changed the username and the password, when registering a user via REST service I'd need to pass these username/password to get a successful result.
• Given either Username and Password (only both) are empty, I should be able to use REST services without a need to pass authentication parameters.