crypto/internal/hpke: add basic implementation

crypto/internal/hpke: add basic implementation

Only implements the sender role, since that's all we need for
client-side ECH for now.

Change-Id: Ia7cba1bc3bad8e8dc801d98d5ea859738b1f2790
Reviewed-on: https://go-review.googlesource.com/c/go/+/585436
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>