Implement MetadataCombiner, partitial refactoring on Metadata

[Wh1isper]

Description

Motivation and Context

• Rename tests/metadata to tests/data_models
• Intro RelationshipInspector for building relations.
• MetadataCombiner framework
• Added generic interface to Metadata

How has this been tested?

Types of changes

• Maintenance (no change in code, maintain the project's CI, docs, etc.)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[sweep-ai]

Apply Sweep Rules to your PR?

• Apply: All new business logic should have corresponding unit tests.
• Apply: Refactor large functions to be more modular.
• Apply: Add docstrings to all functions and file headers.

[codecov-commenter]

Codecov Report

Attention: 60 lines in your changes are missing coverage. Please review.

Comparison is base (00ba7f1) 75.68% compared to head (2596997) 76.63%.

Files	Patch %	Lines
sdgx/data_models/combiner.py	51.51%	48 Missing ⚠️
sdgx/data_models/metadata.py	81.81%	12 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
+ Coverage   75.68%   76.63%   +0.95%     
==========================================
  Files          58       59       +1     
  Lines        2274     2435     +161     
==========================================
+ Hits         1721     1866     +145     
- Misses        553      569      +16     

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[Wh1isper]

Still remaning work:

• Adding mock data for multiple tables
• Adding unit tests for MetadataCombiner.

[MooooCat]

Looks good to me.

(Maybe we should add more demo data for multi-table scenario, I can start a issue and work on it.)

[Wh1isper]

Maybe we should add more demo data for multi-table scenario, I can start a issue and work on it.

I'll be introducing some very simple mock data to use as unit tests, @MooooCat please help us gather some real multi-table scenario data to introduce into the project, like the current adult.csv.