Allow the selection of custom fork of technology-data

[koen-vg]

Sometime like this seems to be necessary in order to use pypsa-eur with technology-data using a different configuration than default; for instance, using 2023 euros instead of 2020 euros ;)

Feel free to suggest alternative ways of doing this if my proposed solution seems a bit ad-hoc.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.