Prompt for cloud storage credentials on session start

[ciyer]

Testing

To test, you need to access to cloud storage that requires authentication (e.g., a private S3 bucket).

• create a (Renku 2) project
• add the private cloud storage
• add a session launcher

Then, when you try to launch a session, you will be prompted to provide the credentials.

/deploy renku=ciyer/release-0.53.2 renku-notebooks=master

[RenkuBot]

You can access the deployment of this PR at https://renku-ci-ui-3203.dev.renku.ch

[RenkuBot]

Tearing down the temporary RenkuLab deplyoment for this PR.