Add retries to downloads of Windows dependencies

[emmatyping]

generate_sbom.py downloads sources to verify their hashes. Sometimes these downloads flake, like in https://github.com/python/cpython/actions/runs/15118338626/job/42494602073?pr=134253

I've made a PR to add retries with exponential backoff:

Linked PRs

• gh-134262: Add retries to generate_sbom.py #134263
• [3.14] gh-134262: Add retries to generate_sbom.py (GH-134263) #134460
• gh-134262: increase retries in Tools/build/generate_sbom.py #134558
• gh-134262: Add retries to downloads in PCbuild\get_external.py #134820
• [3.14] gh-134262: Add retries to downloads in PCbuild\get_external.py (GH-134820) #134865
• [3.13] gh-134262: Add retries to downloads in PCbuild\get_external.py (GH-134820) #134866
• gh-134262: Fix off by one errors in download retry functions #134867

[emmatyping]

Re-opening since the underlying issue isn't resolved yet.