🟥 Add support for `requirements.txt`-style extra dependencies

[julian-risch]

• 🟥 To evaluate if each pipeline should run in separate environment

[mpangrazzi]

I would like to add a note on this: I am not sure this will still be a big requirement. Running each pipeline in a dedicated venv can make things way more complex code-wise, and we may not have enough data to actually decide on this (I feel that most of the people deploy 1-2 pipelines, so this change will add a lot of overhead).

I think it's generally easier to e.g. build a custom Docker image starting from Hayhooks one and deploy it somewhere.