Update 1password-cli: add gpg stanza (#38398)

Even though we’re not quite there yet regarding GPG support (cf.
issue #5971 and PR Homebrew/brew#1335), I’d
still prefer for new casks to have `gpg` stanzas where detached
signatures are available (cf. PR #6185).

For details on the authenticity of the public key, see:

- https://support.1password.com/command-line-getting-started/#set-up-the-command-line-tool

- https://keybase.io/1password

Casks/1password-cli.rb

@@ -8,6 +8,7 @@
           checkpoint: 'a0c73938742c33399b0d408dac38e5aaa23a56e8a76ba0603876398f7cc38ccb'
   name '1Password CLI'
   homepage 'https://support.1password.com/command-line/'
+  gpg 'op.sig', key_url: 'https://keybase.io/1password/pgp_keys.asc'
 
   binary 'op'