Add plugins to 4.0 branch

[bvdmitri]

This PR adds plugins to the codebase of GraphPPL. The PR also introduces major structural changes to the codebase with the purpose of better performance and memory efficiency. Adds more tests and, in principle, should make it easier to integrate the codebase with ReactiveMP.

Thanks for tests @wouterwln :)

I didn't touch the documentation, so it probably is not really valid in some places, but we can fix it in the 4.0.0 branch.

[codecov]

Codecov Report

Attention: 66 lines in your changes are missing coverage. Please review.

Comparison is base (56609a5) 92.44% compared to head (efae863) 90.26%.
Report is 1 commits behind head on dev-4.0.0.

Files	Patch %	Lines
src/graph_engine.jl	85.08%	51 Missing ⚠️
...onal_constraints/variational_constraints_engine.jl	94.48%	8 Missing ⚠️
src/plugins/meta/meta.jl	73.33%	4 Missing ⚠️
src/model_macro.jl	93.33%	2 Missing ⚠️
src/plugins_collection.jl	95.65%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           dev-4.0.0     #153      +/-   ##
=============================================
- Coverage      92.44%   90.26%   -2.19%     
=============================================
  Files              9       12       +3     
  Lines           1416     1561     +145     
=============================================
+ Hits            1309     1409     +100     
- Misses           107      152      +45     

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[bvdmitri]

Lets proceed and fix the documentation in the 4.0.0 branch