fix: use HiGHS to parse CBC sol correctly

[siddharth-krishna]

Closes # (if applicable).

Changes proposed in this Pull Request

CBC's solve_problem_from_file method was assuming that all variables start with x which is not true when calling the solver on a pre-existing MPS file. This PR uses highspy to parse the input file and get the set of variable names, which can then be used as a more accurate filter when parsing CBC's solution file.

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.