feat: email linking for SSO

[mubbsharanwar]

Make email linking for SSO as default behavior.

Steps to test locally:
Enable social-auth locally for SSO.
Create account using registration form flow.
SSO through same email which user used for first time registration.
User will redirect to LMS after authentication.
User account will be linked to existing account.

VAN-331

[waheedahmed]

I guess we can remove this setting override now from the test case since it's enabled by default.
https://github.com/openedx/edx-platform/blob/1df2c691db91e25ef02efee330e642734245b9f9/common/djangoapps/third_party_auth/tests/specs/base.py#L792-L793

[mubbsharanwar]

@waheedahmed Oh yes we can remove this override setting because it's enabled by default now. I updated PR plz verify changes and let me know if there is any further change required.

[pdpinch]

It doesn't look like the JIRA issue https://openedx.atlassian.net/browse/VAN-331 is open to the public. Can you add some more details to the PR description and/or the commit message about what this feature does?

[mubbsharanwar]

Hi @pdpinch,
Currently, in the edX platform, if you try to login with an SSO account using the login form, a username is generated for you based on the email that is used with the SSO provider.

If the username already exists in the platform, you run into a small conflict which sends you back to the login form and requests to sign in using password.
It basically requests that you link your edX account and your SSO account.

In this PR we just try to reducing the user hurdle and make it default behavior to link your edX account and your SSO account by removing ENABLE_REQUIRE_THIRD_PARTY_AUTH flag from pipeline because a pipeline was added to associate the user by email when the ENABLE_REQUIRE_THIRD_PARTY_AUTH flag enable and the social auth provider is an Oauth2 provider. But by default ENABLE_REQUIRE_THIRD_PARTY_AUTH was disabled and we removed this flag from pipeline to link edX account and your SSO account.

[waheedahmed]

Hi @pdpinch, this functionally was added by the community in this PR https://github.com/edx/edx-platform/pull/25935 but behind the ENABLE_REQUIRE_THIRD_PARTY_AUTH flag and we have enabled it by default for all.

[edx-pipeline-bot]

EdX Release Notice: This PR has been deployed to the staging environment in preparation for a release to production.

[edx-pipeline-bot]

EdX Release Notice: This PR has been deployed to the production environment.