feat(JD-1/DS-314): Microsite aware key-secret pairs for oauth

[Henrrypg]

Description

This PR allows us to create an Oauth2 configuration for each site with different backends, enabling multitenancy.

A more detailed description of these changes: #433

How to test

1. Add backends to your platform settings
   File: env/apps/openedx/settings/lms/development.py

"THIRD_PARTY_AUTH_BACKENDS": [
        'social_core.backends.google.GoogleOAuth2',
        'social_core.backends.linkedin.LinkedinOAuth2',
        'social_core.backends.facebook.FacebookOAuth2',
        'social_core.backends.azuread.AzureADOAuth2',
        'third_party_auth.saml.SAMLAuthBackend',
        'third_party_auth.lti.LTIAuthBackend',
]

2. Enable TPA and add third party auth as installed app in your tenant:

"FEATURES": {
	.
	.
	.
	"ENABLE_THIRD_PARTY_AUTH": true
    },

"EDNX_TENANT_INSTALLED_APPS": [ "common.djangoapps.third_party_auth" ]

3. Create an Oauth2 Configuration (using Google backend) for Tenant A and Tenant B:
   

Checkout this document on how to configure your Google provider

4. List Oauth2 Configurations:

Before changes (optional)

With JD-1

5. Login/register to your site using Google

If your site/provider is well configured then you'll be able to register and login to your site.

[JuanDavidBuitrago]

It's working as expected

[MaferMazu]

It works, but I think it is important you update the documentation in favor of tenant configs instead of a microsite; the variable ENABLE_THIRD_PARTY_AUTH should be inside FEATURES (the doc doesn't reflect that); and in the PR you mention stack-builder, I think you can drop it because that setting can be placed in that file only with Tutor if you want. Can you update the documentation? Thanks in advance @Henrrypg ✨

[Henrrypg]

Thank you @MaferMazu, the doc and the PR is updated.