add download endpoint for rustdoc archive

[syphar]

This resolves #174 for offline documentation readers.

The tag specifications & static domain were already set up with @Mark-Simulacrum , see here.

A working download can be seen here, while other crates / versions would get a 403 / forbidden error.

note for deploy:

we need to check with @pietroalbini if the access key on the docs.rs server has the needed permissions:

• s3:PutObjectTagging on the bucket
• create-invalidation for the static.docs.rs cloudfront distribution

[syphar]

r? @Nemo157 @jyn514

[syphar]

also I imagine in the future we would also run the JSON-build and offer a download too.

[syphar]

invalidation is incomplete, I would like to fix it when #1864 is reviewed & merged & this PR is rebased on it.

I would love a review on this PR anyways

[pietroalbini]

Tweaked the permissions.

[syphar]

after #1864 was merged & deployed I rebased & updated this PR.

So this would be ready for review, ping @Nemo157 @jsha ?

Any comments about the design @jyn514 ?

[syphar]

@Nemo157 I updated the branch according to your comments. Ready for another round :)

[Nemo157]

I've tested this locally with minio and the download is working and I can see the tag being applied when I hit the download endpoint:

> mcli tag list docsrs/rust-docs-rs/rustdoc/bpaf/0.6.1.zip
Name                     : http://localhost:9000/rust-docs-rs/rustdoc/bpaf/0.6.1.zip ()
static-cloudfront-access : allow