http2: Allow reading frame header and body separately

This change exports two new methods on the Framer, ReadFrameHeader and
ReadFrameBodyForHeader, which split the functionality of the existing
ReadFrame method.

This provides more granular control, allowing callers to inspect the
frame header before deciding whether or how to read the frame body.
This is useful for applications that may need to make decisions based on frame
type.

Fixes golang/go#73560

Change-Id: I60b42d2889095fac8e243022886740bc6dd94012
Reviewed-on: https://go-review.googlesource.com/c/net/+/710515
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>