Update bridge optimizers with variable bridges

[blegat]

Extracted from #759

[mlubin]

I don't have any substantive comments, so merge when ready.

[codecov-io]

Codecov Report

Merging #816 into master will decrease coverage by 1.19%.
The diff coverage is 54.75%.

@@            Coverage Diff            @@
##           master     #816     +/-   ##
=========================================
- Coverage   92.04%   90.84%   -1.2%     
=========================================
  Files          65       66      +1     
  Lines        7218     7431    +213     
=========================================
+ Hits         6644     6751    +107     
- Misses        574      680    +106

Impacted Files	Coverage Δ
src/Bridges/Variable/Variable.jl	100% <ø> (ø)	⬆️
src/Bridges/Variable/single_bridge_optimizer.jl	0% <0%> (ø)
src/Bridges/Constraint/slack.jl	93.75% <100%> (+0.2%)	⬆️
src/Bridges/Constraint/single_bridge_optimizer.jl	44.44% <20%> (-15.56%)	⬇️
src/Bridges/bridge_optimizer.jl	62.21% <55.64%> (-31.46%)	⬇️
src/Bridges/lazy_bridge_optimizer.jl	65.16% <58.66%> (-32.27%)	⬇️
src/Bridges/Variable/bridge.jl	0% <0%> (ø)	⬆️
src/Utilities/mockoptimizer.jl	90.14% <0%> (+0.49%)	⬆️
src/Bridges/Constraint/quad_to_soc.jl	97.36% <0%> (+1.31%)	⬆️
... and 14 more

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