x/crypto/openpgp: Could EncryptedKey.Decrypt be extended to help support keys kept in a hardware module

[ragansa]

For all the same points made in: #15841
And similar to the change made in: https://go-review.googlesource.com/c/crypto/+/23802/

https://github.com/golang/crypto/blob/master/openpgp/packet/encrypted_key.go#L80-L86
Uses concrete implementations, could we instead take the decrypter interface instead?

Specifically, we're using a hardware backed RSA key and want to use it for PGP decryption.

Thanks in advance!

[ragansa]

cc @ivmaykov

[gopherbot]

Change https://golang.org/cl/189497 mentions this issue: allow using custom decryptor in openpgp RSA decrypt path