Add support for multiple solutions

[odow]

This is a WIP PR to add multiple objective support to JuMP.

Currently blocked by changed needed in MOI.

This PR adds support for querying multiple results form solvers via result_index.

[codecov]

Codecov Report

Merging #2100 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2100      +/-   ##
==========================================
+ Coverage   90.95%   91.09%   +0.14%     
==========================================
  Files          38       40       +2     
  Lines        4321     4402      +81     
==========================================
+ Hits         3930     4010      +80     
- Misses        391      392       +1

Impacted Files	Coverage Δ
src/variables.jl	95.05% <100%> (ø)	⬆️
src/JuMP.jl	82.11% <100%> (+1.13%)	⬆️
src/aff_expr.jl	88.54% <100%> (ø)	⬆️
src/optimizer_interface.jl	81.81% <100%> (+0.42%)	⬆️
src/nlp.jl	92.1% <100%> (+0.16%)	⬆️
src/objective.jl	91.17% <100%> (ø)	⬆️
src/constraints.jl	91.6% <100%> (ø)	⬆️
src/quad_expr.jl	94% <100%> (ø)	⬆️
src/indicator.jl	96% <0%> (ø)
src/callbacks.jl	100% <0%> (ø)
... and 2 more

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