Add KADABRA betweenness centrality approximation algorithm

[DerSchmachtin]

Description

This Pull Request introduces the KADABRA (K-ADaptive Approximation of Betweenness centrAlity) algorithm to Graphs.jl.

Computing exact betweenness centrality can be prohibitively expensive on very large graphs. KADABRA provides a fast and highly scalable randomized approximation of the betweenness centrality, as well as an efficient method to find the top-k nodes with the highest betweenness.

The implementation includes two main exported functions:

• kadabra_centrality: Approximates the betweenness centrality for all vertices.
• kadabra_top_k: Returns the top k vertices with the highest betweenness centrality.

Reference:

Borassi M., Natale E. (2016) KADABRA is an ADaptive Algorithm for Betweenness via Random Approximation. In: ESA 2016. [https://dl.acm.org/doi/10.1145/3284359]

Checklist

• Added kadabra.jl to src/centrality/
• Added corresponding test suite in test/centrality/kadabra.jl and registered it in runtests.jl
• Code has been formatted using JuliaFormatter.jl according to BlueStyle
• Added documentation strings and exported the functions in Graphs.jl
• Ensured RNG scoping follows Graphs.jl conventions using standard library imports

All tests pass locally. Let me know if there are any changes or further optimizations you would like me to make!

Benchmarks & Performance

To verify the performance, I ran some benchmarks comparing this Julia implementation against the original, highly optimized C++ version from the authors.

Hardware Setup:
All benchmarks were executed on an AMD Ryzen Threadripper 3960X 24-Core Processor (3.80 GHz, 48 threads) with 125 GiB of RAM running Ubuntu 22.04 LTS.

The Julia implementation is competitive compared to the Original C++ Implementation.
Here are the results (using k=0, delta=0.1 and epsilon=0.0001) on a few test instances taken from SNAP:

[codecov]

Codecov Report

❌ Patch coverage is 99.05660% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.57%. Comparing base (356aa72) to head (e213fd1).

Files with missing lines	Patch %	Lines
src/centrality/kadabra.jl	99.05%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #518      +/-   ##
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+ Coverage   97.46%   97.57%   +0.11%     
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  Files         127      129       +2     
  Lines        7766     8341     +575     
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+ Hits         7569     8139     +570     
- Misses        197      202       +5     

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