http2: close connections when receiving too many headers

Maintaining HPACK state requires that we parse and process
all HEADERS and CONTINUATION frames on a connection.
When a request's headers exceed MaxHeaderBytes, we don't
allocate memory to store the excess headers but we do
parse them. This permits an attacker to cause an HTTP/2
endpoint to read arbitrary amounts of data, all associated
with a request which is going to be rejected.

Set a limit on the amount of excess header frames we
will process before closing a connection.

Thanks to Bartek Nowotarski for reporting this issue.

Fixes CVE-2023-45288
Fixes golang/go#65051

Change-Id: I15df097268df13bb5a9e9d3a5c04a8a141d850f6
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/2130527
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
Reviewed-on: https://go-review.googlesource.com/c/net/+/576155
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>