[Architecture] Changes to the `.rpa` file format once generic deployments are supported

[marstamm]

This Issue is a copy of https://github.com/camunda/product-hub/issues/2710#issuecomment-2912140827 to ensure we have proper tracking.

This Issue is blocked by camunda/camunda#20758

Migration strategy for generic resource deployments

Given the shortcomings of embedding Resources directly into JSON, we should explore options when generic resource deployments (camunda/camunda#20758) becomes available.

Architecture changes to the RPA file format:

• Linking is done with relative file paths
• New resources can live anywhere in the project folder
• We keep the same editor interface. Additional resources still need to be explicitly added to the .rpa file, opposed to everything in the project being available by default. This also enables efficient reuse of the same resources across different RPA files, as long as they are all deployed in the same process application
• We link all resources, including main.robot. RPA files now become pure Metadata files. This enables better diffing for Github versioning.

Changes for existing rpa files, that use embedded files (#206):

• We transparently extract embedded resources into .camunda/<script_id>/