acme/autocert: add an option for RSA-based certs

Currently, autocert.Manager always generates EC-based certificates.
This change adds an optional field forcing the Manager to use RSA
instead.

An alternative idea, a "double" certificate, where the Manager
presents either RSA or EC certificate based on client's compatibility,
doesn't seem to be worth the implementation time given the constant
increase in Elliptic Curve cryptography.

Fixes golang/go#17744

Change-Id: Idc68abfc698bcff4aad99715baefc06f8fae50ad
Reviewed-on: https://go-review.googlesource.com/34570
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed