contrib.solver.highs

[michaelbynum]

Changes proposed in this PR:

• Port the HiGHS interface from appsi to contrib.solver
• Move the treat_fixed_vars_as_params option from a config option to a private attribute that can only be set at construction time. The reason for this is that changing the value of this parameter between solves can result in incorrect results.
• Make it easier to change the "active" config object so the correct options get used in other parts of the class.

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