Add OAuth2 authentication support

[dioubernardo]

This implementation builds upon the existing GitHub authentication code, using it as a base to support a custom OAuth2/SSO server, and has been tested with Keycloak. It also adds the ability to provide custom CA certificates (CA_CERTS) to the HTTP transport.

[Copilot]

Pull request overview

This PR adds OAuth2/SSO authentication support to the Helios voting system, building on the existing GitHub authentication implementation and tested with Keycloak. It also adds support for custom CA certificates in HTTP transport.

• Adds a new generic OAuth2 authentication system that can be configured for custom OAuth2/SSO providers
• Introduces CA_CERTS configuration for custom certificate authority support
• Updates login UI to display custom OAuth provider names

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
settings.py	Adds OAuth2 configuration variables (client ID, secret, URIs, etc.) and CA_CERTS setting; fixes trailing whitespace
helios_auth/views.py	Adds oauth_name context variable to login views for custom OAuth provider display; whitespace cleanup
helios_auth/templates/login_box.html	Updates login UI to conditionally display custom OAuth provider name instead of generic "oauth" label
helios_auth/media/login-icons/oauth.png	Adds OAuth login icon image
helios_auth/auth_systems/oauth.py	New OAuth2 authentication module implementing OAuth2 flow with CA cert support
helios_auth/auth_systems/init.py	Registers the new OAuth authentication system

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