feat: add support for user.signingKey

[ayakovlenko]

users:
  work: &work_user
    name: "John Doe"
    email: "john.doe@corp.com"
    sign:
      key: "<path to ssh key>"
      format: "ssh"

hosts:
  "github.com":
    overrides:
      - owner: "corp"
        user: *work_user

Set:

git config --local commit.gpgsign true

git config --local user.signingKey "<path to ssh key>"

git config --local  gpg.format ssh