Support GPG "server" configuration for passphrase

[chrisvest]

Is your feature request related to a problem? Please describe.
The maven-gpg-plugin can take the private key passphrase from a server configuration in settings.xml.
See: https://maven.apache.org/plugins/maven-gpg-plugin/usage.html#Configure_passphrase_in_settings.xml

Validation currently prevents the given examples from working, first saying that either username or configuration must be provided.
Then, when configuration is provided, the error is that username and password must be provided.
If username is provided then password must also be provided (according to settings.xml docs), but if password is provided then passphrase will supposedly be ignored.

Describe the solution you'd like
Make the validation allow servers that only have id and passphrase.

Describe alternatives you've considered
The alternative is to specify the GPG key passphrase on the command line, and rely on GitHub to filter secret data from the build logs.

[slawekjaranowski]

PR are welcome

[danielflower]

Workaround: give an empty configuration.

            {
              "id": "gpg.passphrase",
              "passphrase": "${{ secrets.OSSRH_GPG_SECRET_KEY_PASSWORD }}",
              "configuration": {}
            }