set & handle etags/if-none-match for first rustdoc assets

[syphar]

Next step towards #1560.

Idea is:

• when we just proxy the asset from S3 directly, we can use the etag we get from the S3 API
• when the asset is fetched out of an archive, we can generate a valid etag based on the full archive etag + the range.

The next step after is trying to figure out an etag for rustdoc pages, which finally could give us the gains laid out in #1560 .

I feel like there is also some duplication around conditional-get handling, but I want to see the rustdoc part first before I start reducing the duplication.

I changed the format of the Last-Modified header too, it looks like using UTC is against RFC, and GMT would be correct.

[syphar]

@GuillaumeGomez have you seen this one here?

[GuillaumeGomez]

Now I did. :)