Add a CI script that generates "all" extra group

[vdusek]

Write a CI script that:

• Reads existing extras,
• Merges them into one list,
• Deduplicates the entries, and
• Writes the "all" extra in pyproject.toml.

We could utilize tomlkit here (it leaves the whitespaces & comments, unlike toml.

[vdusek]

but of course, maybe it's not worth it 🙂

[janbuchar]

It could also be a CI check and updating the all group would be up to the PR author - that would probably be less error prone