Modify inference for predictions functionality

[albertpod]

This PR aims at bringing the prediction functionality into RxInfer.jl.

[albertpod]

Short summary of the modifications:

• The functions make_actor have been modified to accept AbstractVariable instead of RandomVariable.
• The InferenceResult struct has been expanded to include a new field named predictions.
• New functions __inference_check_dataismissing, __inference_fill_predictions have been added to handle predictions.
• The inference function has been updated to handle predictions.
• The data argument is now optional if predictvars are specified.
• New logic has been added to handle prediction variables.
• Subscription logic has been updated to handle both types of variables.
• Several error checks and warnings have been added or modified to ensure that the data and prediction variables are correctly specified and handled.

[albertpod]

NOTE:
The prediction functionality doesn't allow free energy computation at the moment.
I have error when free_energy and predictions are both present.
The research question remains: how do we compute FE on non-terminated graph (ping @ismailsenoz )

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Codecov Report

Patch coverage: 76.71% and project coverage change: -0.29% ⚠️

Comparison is base (2ba4dea) 81.32% compared to head (c309034) 81.03%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
- Coverage   81.32%   81.03%   -0.29%     
==========================================
  Files          11       11              
  Lines        1210     1271      +61     
==========================================
+ Hits          984     1030      +46     
- Misses        226      241      +15     

Files Changed	Coverage Δ
src/inference.jl	75.77% <74.60%> (+0.07%)	⬆️
src/model.jl	85.85% <90.00%> (+0.14%)	⬆️

... and 1 file with indirect coverage changes

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