Add other oauth providers

[d4alpha]

Feature request

Currently only Github, Gitlab and BitBucket Cloud are supported Oauth providers. It would be great if any OAuth provider, for example Keycloak or Auth0 could be used.

Use case

When using another Git service than Github, Gitlab and BitBucket Cloud, it is not possible to use the authenticated part of Tekton Hub. By supporting other OAuth providers the authentication could be solved independent of the Git provider.