Enhance functional dependencies customization in ReactiveMP

[bvdmitri]

This PR fixes #436

This pull request include updates to the ReactiveMP library documentation and functionality, focusing on the customization of functional dependencies for nodes. The changes enhance the documentation, add new features for runtime customization, and includes tests to ensure the new functionality works as intended.

Users now can specify

function ReactiveMP.collect_functional_dependencies(::Type{CustomNode}, options::FactorNodeActivationOptions)
    # custom behaviour
end

to set functional dependencies in runtime. options has an access to various settings, including metadata as requested in the original issue.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.23%. Comparing base (9368139) to head (a3a943d).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nodes/nodes.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #437      +/-   ##
==========================================
+ Coverage   72.56%   73.23%   +0.66%     
==========================================
  Files         190      190              
  Lines        5493     5495       +2     
==========================================
+ Hits         3986     4024      +38     
+ Misses       1507     1471      -36     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.