Add http2 feature to allow disabling HTTP/2 support

[jirutka]

The HTTP/2 support requires the h2 crate which is quite a heavy dependency.

Resolves #1137

All tests are green: https://github.com/jirutka/reqwest/actions/runs/3222094053 (requering approval to run CI jobs in pull requests from new contributors is quite annoying default).

[seanmonstar]

This would indeed be a good addition, but it's a breaking change, so we need to bundle it with the next breaking release (not yet scheduled, but will follow hyper 1.0).

[jirutka]

When do you expect that to happen? Are we talking about weeks, months or years?

[seanmonstar]

hyper 1.0 is a couple months away. After that, updating reqwest will happen soon after.

[jirutka]

What if I revert the logic – add no-http2 feature to disable http2 when enabled? We can switch it back before releasing 1.0.

[seanmonstar]

Adding a feature like that can cause breakage. If anyone in the dependency tree enabled the feature, it would disable HTTP/2 for any other crate that expected it to work.

[jonas-schievink]

FYI this breaks cargo build --features blocking --no-default-features, looks like some #[cfg]s in blocking.rs are missing.

[jonas-schievink]

FYI, we've found a bug in this PR, where reqwest will fail to communicate with a hybrid HTTP1+2 server with TLS enabled when reqwest is built with HTTP2 disabled. This seems to happen when doing ALPN negotiation, so presumably all of the HttpVersionPref::All branches should be #[cfg]d to do tls.request_alpns(&["http/1.1"]); instead of the current tls.request_alpns(&["h2", "http/1.1"]);?

[seanmonstar]

This will be part of 0.12, and was merged in #2162.