This adds two new subcommands (gpg-key list and gpg-key add) based off of the ssh-key subcommands. I've never worked with Go before, so hope this is correct.

Related to #1755 (comment)

$ gh gpg-key add ~/gpg_key.asc
✓ GPG public key added to your account

$ gh gpg-key
Manage GPG keys registered with your GitHub account

USAGE
  gh gpg-key <command> [flags]

CORE COMMANDS
  add:        Add a GPG key to your GitHub account
  list:       Lists GPG keys in your GitHub account

Currently gh gpg-key list will show the output using the following columns:

• GPG key ID
• Expiry date
• Public key ID (truncated in the middle if necessary)

I wasn't sure whether to included the date it was uploaded, or anything else as it seemed to get a bit cluttered.

I've temporarily created a GitHub CLI extension for this, however I'd much rather it be in core. 👍🏻