Option to extend solid biomass potentials by imports

[millingermarkus]

Closes # (if applicable).

Changes proposed in this Pull Request

Added solid biomass imports with a toggle and settings for price, max amount and upstream emissions

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.