Add Knitro as a new supported solver

[finozzifa]

Closes # (if applicable).

Changes proposed in this Pull Request

This pull request adds Knitro as a new solver supported by linopy.

The supporting documentation for Knitro is available at this link.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[FabianHofmann]

supersed by #532