crypto/ecdh: add KeyExchanger interface

crypto/ecdh: add KeyExchanger interface

Updates #75300

Change-Id: I6a6a6964bbfa1f099c74d0a3fb3f7894d7b1b832
Reviewed-on: https://go-review.googlesource.com/c/go/+/705795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>