exploration: replace gpgme with rpgp

[Alizter]

This is a way to replace gpgme with rpgp. I haven't actually tested it so it might be incorrect. I haven't bothered to update the errors just yet, but @drewdeponte you will probably want to do that himself anyway.

[Alizter]

@drewdeponte I've pushed another commit here that tests gps ls in the way I have been talking about as an example. I don't have time to finish setting up the gpg signing just yet, but you could take this methodology and use it elsewhere. gps ls is done for example.

[drewdeponte]

@Alizter I briefly looked into this last night and it seems like the first step is just figuring out how to get the private key we need out of gpg given a key id. Because what you give to the git config in the user.signingKey property is your GPG key id.

So, I tried looking at the library, but I don't think it provides anything for reading a GPG keyring and getting a key out. So, I think we would need to externally run some gpg command to output the key to stdout, and read it in that way.

I am not sure what this gpg command would be exactly. But I think that is the next piece of research to help move this along if you want to search around.

[drewdeponte]

Actually, this might be what we want gpg --armor --export-secret-key <key-id>. I am not sure 100%.

[drewdeponte]

Thinking more about this, I think we can get rid of entire rpgp dependency as well if we just shell out to the command to do the signing. I believe this is what Git actually does. It streamlines the dependencies we have to worry about across platforms and because we are piggy backing off of the Git config we can get the info about the gpg command from that or fall back to looking for it in the path.

[drewdeponte]

After some quick googling this might be helpful, https://unix.stackexchange.com/questions/475052/how-to-detach-sign-a-file-with-a-specific-private-key-why-this-fails

[drewdeponte]

I am closing this PR because I just integrated commit ec71fc2 which replaces the nead for this as it implements gpg signing using the external gpg command.