net/http: add StatusTooEarly (425)

StatusTooEarly can be returned to indicate that a server is unwilling
to accept early data as introduced in TLS 1.3.
The status code was specified in RFC 8470, section 5.2.

Major supported browsers are:
- Firefox as of version 58
  https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/425#Browser_compatibility
- Chromium as of version 73.0.3628.1
  https://chromium.googlesource.com/chromium/src/+/58097ec3823e0f340ab5abfcaec1306e1d954c5a

Change-Id: I3f62f4193bae198994d08fde7e92e0ccd080e59a
GitHub-Last-Rev: fa885040eaf80e0e33b571567108d8a9ded67801
GitHub-Pull-Request: golang/go#29073
Reviewed-on: https://go-review.googlesource.com/c/152118


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>