Add AWS region IP size script

[Yassinbrine]

Summary

• adds aws_region_ip_sizes.py to fetch AWS ip-ranges.json
• collapses overlapping IPv4 CIDRs with the standard library before counting addresses by region
• documents the helper near the existing IP-addresses-per-region section

Fixes #11

Bounty

Submitting under PatMyron's standing bounty program. PayPal: https://paypal.me/ShaimaaElkadi

Testing

• python -m py_compile aws_region_ip_sizes.py
• local sample check for overlapping CIDRs: 10.0.0.0/24 plus 10.0.0.0/25 counts as 256, not 384
• git diff --cached --check

I did not run the live AWS download from this environment.