This PR adds the ability to optionally disable/make uneditable the registration form fields when using third party authentication. This feature can be enabled for any provider by adding `PROVIDER_READ_ONLY_FIELDS` options under `Other Settings` (incase of OAuth2) or `Advanced Settings` (incase of SAML), which contains the list of fields to be made read-only.
*Sandbox URL:* TBD
*Author notes and concerns:*
Currently it is assumed that only the default registration fields (i.e. name, email and password) are used. Incase additional fields are used, the [restrictions](https://github.com/open-craft/edx-platform/blob/8457746a05f10b107a247b6f09873b58db8c48b9/openedx/core/djangoapps/user_api/helpers.py#L126-L133) should be updated accordingly.