x/crypto/openpgp: Implement opaque DSA keys

[Merovius]

I'm writing a package to use gpg-agent for private key operations, implementing the crypto.Signer and crypto.Decrypter interfaces. Currently, the openpgp package doesn't support that for DSA. This would be analogous to https://go-review.googlesource.com/c/crypto/+/23802

[edit] I just sent out the CL for this one, as it's independent and more important for me to get in soon-ish :) Will file separate issues for other things.

[gopherbot]

Change https://golang.org/cl/137955 mentions this issue: openpgp: Handle DSA crypto.Signer

[FiloSottile]

See #27905