crypto/internal/hpke: modularize API and support more ciphersuites

Updates #75300

Change-Id: I6a6a6964de449b36bc6f5594e08c3c47a0a2f17f
Reviewed-on: https://go-review.googlesource.com/c/go/+/701435
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>