http2: avoid API changes when built with go1.27

The wrapping implementation accidentally dropped some symbols.
Preserve them.

Move errors into common locations:
  - ErrNoCachedConn
  - ErrPushLimitReached
  - ErrRecursivePush
  - GoAwayError

Move one constant into a common location:
  - TrailerPrefix

Preserve methods of FrameWriteRequest as no-ops.
These are not a user-serviceable part and there is no way
for users to construct FrameWriteRequests.
  - FrameWriteRequest.Consume
  - FrameWriteRequest.DataSize
  - FrameWriteRequest.StreamID
  - FrameWriteRequest.String

Preserve functions which create write schedulers,
returning a no-op implementation of WriteScheduler:
- NewPriorityWriteScheduler
- NewRandomWriteScheduler
- PriorityWriteSchedulerConfig

For golang/go#78508

Change-Id: I327603137cf69d93bb405d9a95e038886a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/776180
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>