Add explainers for nearest neighbor models

[Zaphoood]

Motivation and Context

This PR adds three new explainers for efficiently explaining sklearns KNearestNeighbor and RadiusNeighborClassifier models.

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Public API Changes

• No Public API changes
• Yes, Public API changes (Details below)

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How Has This Been Tested?

Unittests have been added that compare the results of the explainers' efficient algorithms to baseline values which are obtained by computing the exact Shapley Values of a ground-truth Game.

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Checklist

• The changes have been tested locally.
• Documentation has been updated (if the public API or usage changes).
• An entry has been added to CHANGELOG.md (if relevant for users).
• The code follows the project's style guidelines.
• I have considered the impact of these changes on the public API.

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[codecov]

Codecov Report

❌ Patch coverage is 90.99437% with 48 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/shapiq/explainer/nn/weighted_knn.py	91.13%	14 Missing ⚠️
src/shapiq/explainer/nn/games/wknn.py	88.33%	7 Missing ⚠️
src/shapiq/explainer/nn/threshold_nn.py	90.90%	5 Missing ⚠️
src/shapiq/explainer/nn/base.py	92.85%	4 Missing ⚠️
src/shapiq/explainer/nn/games/base.py	91.66%	4 Missing ⚠️
src/shapiq/explainer/nn/games/tnn.py	90.32%	3 Missing ⚠️
src/shapiq/explainer/nn/knn.py	92.30%	3 Missing ⚠️
src/shapiq/explainer/nn/_util.py	89.47%	2 Missing ⚠️
src/shapiq/explainer/nn/games/_util.py	88.88%	2 Missing ⚠️
src/shapiq/explainer/nn/games/knn.py	89.47%	2 Missing ⚠️
... and 1 more

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