feat: redirect to custom URL when third-party auth account is unlinked

[ArturGaspar]

Description

Backport of openedx#1078

Allow redirecting to a custom URL when signing in via third-party auth when the account is not linked.

How Has This Been Tested?

1. Set the following settings in the LMS:
   • ENABLE_MFE_CONFIG_API = True
   • ENABLE_COMBINED_LOGIN_REGISTRATION = True
   • ENABLE_THIRD_PARTY_AUTH = True
   • AUTHN_MICROFRONTEND_URL = 'http://localhost:1999'
   • MFE_CONFIG = {"TPA_UNLINKED_ACCOUNT_PROVISION_URL": "http://example.com"}
2. Set 'ENABLE_THIRD_PARTY_AUTH': True and 'ENABLE_AUTHN_MICROFRONTEND': True in FEATURES dict in LMS settings (edx-platform/lms/envs/common.py).
3. Create a dummy backend Oauth2 provider at http://localhost:18000/admin/third_party_auth/oauth2providerconfig/ and enable its "Visible" setting.
4. Set MFE_CONFIG_API_URL='http://localhost:18000/api/mfe_config/v1' in frontend-app-authn (e.g. in .env.development).
5. Go to http://localhost:1999/login.
6. Sign in with the "Dummy" provider.
7. See that you are redirected to example.com

The login page will briefly show before redirect but I see no way to prevent this as the request to http://localhost:18000/api/mfe_context is made asynchronously and we don't know the third-party auth context until it is made, but the login page can be rendered before that.

Merge Checklist

• If your update includes visual changes, have they been reviewed by a designer? Send them a link to the Sandbox, if applicable.
• Is there adequate test coverage for your changes?

Post-merge Checklist

• Deploy the changes to prod after verifying on stage or ask @openedx/vanguards to do it.
• 🎉 🙌 Celebrate! Thanks for your contribution.

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