drpchttp: add helpers for ALPN support

[hdonnay]

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I was looking at using DRPC and would want to run it on the same server connection as an existing API. ALPN is the "native" way to do this, so I took a pass at implementing it. Sending a PR in case it's something that would be useful for upstream.

I couldn't find if there was a Gerrit instance this sort of thing should go to instead.

[hdonnay]

Here's the rendered documentation for the added bits. (Looks like firefox mangled it a bit, sorry.)

[zeebo]

I finally checked my emails and saw this PR. I haven't taken a detailed look yet (I'll try to do that tomorrow), but I think this might live better in the drpcmigrate package (which is where we have code to do muxing based on some header prefix at the start of the data). The drpchttp package is more about serving drpc endpoints over http.

Also we do have gerrit at https://review.dev.storj.io/ but this works fine as well, whichever you prefer. I should add a link in the main README.