Added support to Elliptic Curve and passphrase-less key generation

[sidneypepo]

Issue # (if available)

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Description of changes

Hello! This simple change adds support for some more Unattended key generation parameters, specifically the (Sub)Key-Curve, which (mainly) enables support for Elliptic Curve key generation, and (Sub)Key-Usage, to select specific (Sub)Key flags. I've also changed the Passphrase parameter in order to support a passphrase-less key generation (but only when it's nil), as GnuPG's default behavior is to ask for a passphrase.

Thanks for such great work at IOStreams!

P.S.: that's literally my first ever written Ruby code, so any feedback around it will be appreciated :)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.