feat: add ReactiveMP.sampletype

[Nimrais]

This PR is solving the ambiguity issue of the eltype inside ReactiveMP.

[codecov-commenter]

Codecov Report

Base: 61.61% // Head: 61.64% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (da38540) compared to base (f9d83ac).
Patch coverage: 85.71% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   61.61%   61.64%   +0.02%     
==========================================
  Files         194      194              
  Lines        7183     7206      +23     
==========================================
+ Hits         4426     4442      +16     
- Misses       2757     2764       +7     

Impacted Files	Coverage Δ
src/distributions/gamma_shape_rate.jl	82.50% <42.85%> (-8.68%)	⬇️
src/distributions/sample_list.jl	91.29% <57.14%> (-0.55%)	⬇️
src/distributions.jl	71.11% <100.00%> (+6.24%)	⬆️
src/distributions/beta.jl	93.33% <100.00%> (ø)
src/distributions/gamma.jl	91.04% <100.00%> (ø)
src/distributions/matrix_dirichlet.jl	93.33% <100.00%> (ø)
src/distributions/normal.jl	89.93% <100.00%> (ø)
src/distributions/pointmass.jl	88.88% <100.00%> (+0.13%)	⬆️
src/nodes/mv_normal_mean_covariance.jl	100.00% <100.00%> (ø)
src/nodes/mv_normal_mean_precision.jl	100.00% <100.00%> (ø)
... and 10 more

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