Initial "dgst" implementation for freebsd and alpine packages.

[oxpa]

I'm using relic to sign a bunch of linux packages. Deb, rpm, apk and (not quite linux:) freebsd pkg.
The two latter use openssl dgst or a wrapper for it
Here are sources for APK: https://github.com/alpinelinux/abuild/blob/292a03128e6c08dae5194f0794fe0d55450171ff/abuild-sign.in#L35
And here is a man page that describes BSD approach to signing repos: pkg-repo

The process is roughly the same: one needs an rsa key and pksc1v1.5 signature. Not quite sure if I'm correct in this naming but Crypto.Signer does exactly what's needed.

The implementation is quite straightforward: just get the key and sign. There is no filetype associated with the signer. Nor there is a way to verify the signature: if I get it right - currently there is no way to pass specific key to use for verification. But openssl pkeyutl does that for me.

Let me know if there is anything I can improve in this code.