Support use of apiKey/apiIssuer instead of username/password

It would be nice if gon supported the use of apiKey/apiIssuer as an alternative to using username and password for notification.
From the help text of altool:
> xcrun altool --help
Copyright (c) 2009-2019, Apple Inc. Version 4.00.1181

Usage: altool --validate-app -f <file> -t <platform> -u <username> {[-p <password>] | --apiKey <api_key> --apiIssuer <issuer_id>}
       altool --upload-app -f <file> -t <platform> -u <username> {[-p <password>] | --apiKey <api_key> --apiIssuer <issuer_id>}
       altool --notarize-app -f <file> --primary-bundle-id <bundle_id> -u <username> {[-p <password>] | --apiKey <api_key> --apiIssuer <issuer_id>} [--asc-provider <provider_shortname>]
       altool --notarization-info <uuid> -u <username> {[-p <password>] | --apiKey <api_key> --apiIssuer <issuer_id>}
       altool --notarization-history <page> -u <username> {[-p <password>] | --apiKey <api_key> --apiIssuer <issuer_id>} [--asc-provider <provider_shortname>]
       altool --list-apps -u <username> {[-p <password>] | --apiKey <api_key> --apiIssuer <issuer_id>}
       altool --store-password-in-keychain-item <name_for_keychain_item> -u <username> -p <password>

Authentication: Most commands require authorization.
                There are two methods available: user name with password, and apiKey with apiIssuer.

                -u, --username <username> Username. Required to connect for validation, upload, and notarization.
                -p, --password <password> Password. Required if username specified and apiKey/apiIssuer are not.
                                          If this argument is not supplied on the command line, it will be read from stdin.
                                          Alternatively to entering <password> in plaintext, it may also be specified using a '@keychain:'
                                          or '@env:' prefix followed by a keychain password item name or environment variable name.
                                          Example: '-p @keychain:<name>' uses the password stored in the keychain password item named <name>.
                                                                         You can create and update keychain items with the
                                                                         --store-password-in-keychain-item command.
                                          Example: '-p @env:<variable>'  uses the value in the environment variable named <variable>

                --apiKey <api_key>        apiKey. Required for JWT authentication while using validation, upload, and notarization.
                                          This option will search the following directories in sequence for a private key file
                                          with the name of 'AuthKey_<api_key>.p8':  './private_keys', '~/private_keys', '~/.private_keys',
                                          and '~/.appstoreconnect/private_keys'.
                --apiIssuer <issuer_id>   Issuer ID. Required if --apiKey is specified.
mitchellh / gon

Support use of apiKey/apiIssuer instead of username/password #7
