Follow-up tasks for BFF

[andreaskienle]

Description

This is a collection of (smaller) follow-up tasks in the context of our BFF:

• Rename src directory to client.
• Investigate and implement js bundler to remove node_modules from Dockerimage.
• Ensure comprehensive TypeScript support across client/BFF.
• Implement testing strategy for client/BFF.
• Create/update README.md for new architecture.
  • Properly document environment variables and their expected format (see comment)
• See if we can validate the environment variables (see comment)
• Clarify and document IdP sign-out flow with BFF.
• Implement /graphql endpoint in BFF.
• Implement environment variable based frontend config served by BFF at /frontend-config.
• Make the FRAME_ANCESTORS env variable optional