feat: support connected services

[leafty]

This PR adds support to mount repositories from external sources in session. This requires connected services support from renku-data-services. Also, the API for starting Renku 2.0 sessions has been modified to accept external repositories.

Changes in notebooks:

• Refactored the UserServer class to be an abstract class. Renku1UserServer and Renku2UserServer inherit from UserServer and contain the corresponding adaptations needed to support Renku 1.0 and Renku 2.0 sessions respectively.
• Changed the amalthea patches to reflect changes in git-clone and git-proxy.
• Updated the patch_statefulset_tokens() method invoked when resuming sessions.

Changes in git-clone:

• Refactored the git-clone container to support cloning repositories from any source.
• Cloning is done optimistically. The git-clone container will not crash if a repository cannot be cloned.
• For private repositories, a git provider must be configured and will be used to clone.
• The container now only uses the renku_access_token.

Changes in git-proxy:

• Refactored the git-proxy container to support injecting credentials from external services.
• The git-proxy is a simple pass-through for anonymous sessions (meaning it should probably not run for anonymous sessions).
• Repositories matching a configured git provider will have credentials injected:
  • From renku-gateway-auth for the internal GitLab
  • From renku-data-services for other services
• The container now only uses the renku_access_token and the renku_refresh_token.

See: SwissDataScienceCenter/renku-data-services#179
This feature can be tested at: https://renku-ci-ds-179.dev.renku.ch/v2.

/deploy renku-data-services=build/add-connected-services-demo renku=build/add-connected-services-demo renku-ui=build/add-connected-services-demo

[RenkuBot]

You can access the deployment of this PR at https://renku-ci-nb-1849.dev.renku.ch