Add NoEncryptionSubkey to openpgp/packet/Config

[dkg]

This is a step toward being able to support

gosop generate-key --signing-only

See https://github.com/ProtonMail/gosop/issues/43

[twiss]

Hey :wave: Thanks for the PR!

I'm a bit hesitant about adding a boolean option that starts with No, essentially making it backwards. Since sop already provides a name for this, how about we borrow from that and name this GenerateSigningKeyOnly or some such?

[dkg]

I agree that the sense inversion is pretty weird. I wanted it to default to false, so that there's nothing extra to do if the struct is 0-initialized. I don't mind naming it SigningOnly if you prefer, but i thought you might want some level of deeper control here (for example, enabling or disabling Authentication-capable subkeys).

If you prefer SigningOnly, i'm happy to revise.

Alternately, instead of adding a member to the struct, i guess we could augment NewEntity with another parameter (though that feels pretty clumsy in terms of API shift, and given golang's opposition to default parameters)