alt-svc: experimental support

[bagder]

This is the alt-svc: parser. This is a prerequisite to allow upgrading to HTTP/3 correctly but can be used for other HTTP versions as well.

Status:

• - read alt-svc file from disk (using documented file format
• - write alt-svc file from disk
• - parse Alt-Svc: response headers, including ma, clear and persist.
• - replaces old entries when new alternatives are received
• - unit tests to verify most of this functionality (test 1654)
• - act on Alt-Svc: response headers
• - build conditionally on configure --enable-alt-svc only, feature marked as EXPERIMENTAL
• - implement CURLOPT_ALTSVC_CTRL
• - implement CURLOPT_ALTSVC
• - document CURLOPT_ALTSVC_CTRL
• - document CURLOPT_ALTSVC
• - document --alt-svc
• - add CURL_VERSION_ALTSVC
• - make curl -V show 'alt-svc' as a feature if built-in
• - support curl --alt-svc [file] to enable caching, using that file
• - make tests/runtests.pl able to filter tests on the feature alt-svc
• - actually use the existing in-memory alt-svc cache for outgoing connections
• - alt-svc cache expiry
• - add tests verifying curl acting on Alt-Svc, from header and from loaded cache

Left to work on after first merge

• handle multiple response headers, when one of them says clear (should override them all)
• using Age: value for caching age as per spec
• CURLALTSVC_IMMEDIATELY support
• CURLALTSVC_ALTUSED support

There are probably a few more details too still missing.

The Alt-Svc spec
Planned curl alt-svc API

[bagder]

Rebased and polished for merge. If this doesn't trigger anything major, I will merge this as EXPERIMENTAL code in a day or two. Disabled by default.