crypto/tls: reorder client_hello extensions

crypto/tls: reorder client_hello extensions

This sets us up for ECH, which need compressible extensions to be
contiguous. Put them at the end to make things easier for everyone.

Change-Id: I2a51f76d5fcd1b6d82325f5a4a8cde6d75f1be0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/585437
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>