Add Bonmin and Couenne to list of supported solvers

[odow]

This took far longer than I expected, but everything is now up and running: https://github.com/jump-dev/AmplNLWriter.jl/runs/2357401205

Using Bonmin or Couenne is as simple as

using JuMP, AmplNLWriter, Bonmin_jll, Couenne_jll

model = Model() do
    AmplNLWriter.Optimizer(Bonmin_jll.amplexe)
end

model = Model() do
    AmplNLWriter.Optimizer(Couenne_jll.amplexe)
end

[codecov]

Codecov Report

Merging #2561 (5d4d85c) into master (d9cd5fb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2561   +/-   ##
=======================================
  Coverage   93.18%   93.18%           
=======================================
  Files          44       44           
  Lines        5489     5489           
=======================================
  Hits         5115     5115           
  Misses        374      374           

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