The HTTP endpoint is `/.git-pages/archive.tar` and it is gated behind
a feature flag `archive-site`. It serially downloads every blob and
writes it to the client in a chunked response, optionally compressed
with gzip or zstd as per `Accept-Encoding:`. It is authorized the same
as `/.git-pages/manifest.json`, for the same reasons.
The CLI operation is `-get-archive <site-name>` and it writes a tar
archive to stdout. This could be useful for an administrator to review
the contents of a site in response to a report.
Both `_headers` and `_redirects` files are present in the output,
reconstituted from the manifest.
