feat: add OAuth 2.0 Client Credentials support for Credly

[luisfelipec95]

Description

This PR adds support for OAuth 2.0 (Client Credentials Grant) authentication for the Credly integration in edx-credentials.

Previously, Credly relied solely on legacy API Keys (Basic Auth). This update enables modern OAuth 2.0 authentication while maintaining full backward compatibility with existing legacy organizations.

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Key Changes

1. Database & Models (credentials/apps/badges/models.py)

• Added oauth_client_id and oauth_client_secret fields to CredlyOrganization.
• Generated and applied database migrations.

2. API Client (credentials/apps/badges/credly/api_client.py)

• Added _get_oauth_token() method to handle the OAuth 2.0 token exchange at /oauth/token.
• Implemented Django cache storage for access tokens to prevent unnecessary HTTP requests during token lifespan.
• Configured dynamic header resolution in _get_headers(): uses Authorization: Bearer <token> when OAuth credentials are provided, falling back to Authorization: Basic <legacy_key>.
• Requested badge_templates issued_badges scopes to cover both badge template syncing and badge issuance operations.

3. Django Admin Interface & Forms (admin.py & admin_forms.py)

• Reorganized CredlyOrganizationAdmin using fieldsets to clearly separate OAuth 2.0 Credentials from Legacy API Key Authentication.
• Updated CredlyOrganizationAdminForm.clean() and ensure_organization_exists() to validate OAuth credentials via fetch_badge_templates() (since Credly restricts GET /v1/organizations/{id} under Bearer auth).

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How to Test

1. OAuth 2.0 Credentials Flow:

   • Go to Django Admin -> Credly Organizations -> Add Organization.
   • Fill in UUID, Oauth Client ID, and Oauth Client Secret (leave API Key blank).
   • Save the organization. Verify that it validates credentials and saves successfully.
   • Run the action Sync organization badge templates and ensure templates are fetched without error.

2. Error Handling Check:

   • Enter invalid Client Secret or Client ID.
   • Submit the form. Verify that a clear validation error message appears at the top of the form instead of a 500 error page.

3. Backward Compatibility Check:

   • Create or edit an organization using only the Legacy API Key.
   • Verify that saving and syncing badge templates still works as expected.

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