net/http: add HTTP2Config

net/http: add HTTP2Config

Add a field to Server and Transport containing HTTP/2 configuration
parameters.

This field will have no effect until golang.org/x/net/http2 is updated
to make use of it, and h2_bundle.go is updated with the new http2
package.

For #67813

Change-Id: I81d7f8e9ddea78f9666383983aec43e3884c13ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/602175
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>