Add a workflow to test imports

[anakin87]

One thing that happens often:

• we introduce a dependency in a lazy import block
• we forget to wrap types coming from the optional dependency in forward references in method signatures (hf_output: TextGenerationStreamOutput instead of hf_output: "TextGenerationStreamOutput")
• tests do not catch the error because the test environment contains the optional dependency
• we figure out the error from the nightly failures in tutorials or core-integrations, which also test with haystack main branch but without the optional dependencies.

I remember some attempts to solve this problem in the past (I can't find the issue).
I would like to create a testing workflow, which

• uses the default environment - no optional dependencies
• tries to detect and import modules
• raises an error or exits