Add ICON module to sources

[devsjc]

Adds a new driving actor which implements DWD's ICON model from their Opendata API.

Overview:

• New folder src/nwp_consumer/internal/inputs/icon containing all the API-specific code and unit tests
• New config ICONConfig in src/nwp_consumer/internal/config/env.py
• Updated entrypoint to add icon option to cli insrc/nwp_consumer/cmd/main.py
• Integration tests in src/test_integration now include ICON-specific tests

[peterdudfield]

Looks great