add burr-redirect package for legacy 'burr' PyPI name

[elijahbenizzy]

Summary

Mirrors the Hamilton sf-hamilton-redirect/ pattern. Adds a thin metadata-only burr wheel that just pins apache-burr==<version> (plus every extra), so users on pip install burr (or pip install burr[start], etc.) keep working and get the current apache-burr release transitively.

The legacy burr PyPI listing is currently at 0.40.2 (pre-Apache). Without this redirect, anyone with burr in their requirements never sees apache-burr updates and is stuck on a year-old codebase.

What this PR adds

• burr-redirect/pyproject.toml.template — setuptools-based, name = "burr", dependencies = ["apache-burr==VERSION"], all 28 extras mirrored.
• burr-redirect/build.sh — stamps VERSION into the template, builds wheel + sdist, runs twine check, prints TestPyPI and PyPI upload commands.
• burr-redirect/README.md — "this package has moved" note, points users at apache-burr.
• burr-redirect/.gitignore — ignores generated pyproject.toml, dist/, build/, *.egg-info/.

Excludes (so this stays out of ASF artifacts)

• .rat-excludes: burr-redirect/** so Apache RAT does not scan the metadata-only redirect package.
• pyproject.toml: burr-redirect/** added to [tool.flit.sdist].exclude so the redirect package is not bundled into the apache-burr source tarball. The redirect is a non-ASF artifact and must not appear in any IPMC-voted release.

What this PR does NOT do

• Does not build, sign, or upload any release artifact.
• Does not change anything about apache-burr's release pipeline.
• The legacy burr 0.42.0 wheel is built and uploaded manually after apache-burr 0.42.0 is live on PyPI, so the == pin resolves. The build.sh output enumerates TestPyPI and PyPI commands for that step.

Local verification (already done)

cd burr-redirect && ./build.sh 0.42.0
# → builds dist/burr-0.42.0-py3-none-any.whl + dist/burr-0.42.0.tar.gz
# → twine check: both PASSED
# → wheel is 4 files, metadata only (no source)
# → all 28 extras present, exact apache-burr==0.42.0 pin

Test plan

• CI green (release-validation, tests, lint) — the redirect dir is excluded from RAT and from apache-burr's flit sdist, so should be a no-op for those jobs
• After merge: maintainer uploads burr 0.42.0 to TestPyPI first (verifying with --extra-index-url https://pypi.org/simple/ so apache-burr resolves), then to real PyPI