http2, internal/httpcommon: factor out common request header logic for h2/h3

HTTP/2 and HTTP/3 use the same set of pseudo-headers to represent
requests and responses. Move the http2 package's logic for validating
an http.Request and converting it to a set of pseudo-headers into
internal/httpcommon so it can be shared with HTTP/3.

For golang/go#70914

Change-Id: I80561752e821ccd0da2a811034c44f3f71064434
Reviewed-on: https://go-review.googlesource.com/c/net/+/643780
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Commit-Queue: Damien Neil <dneil@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Damien Neil <dneil@google.com>