Feature Request - Add app credentials

[smarcet]

hello guys , greetings !!! i work for the OSF i do think this feature will be really useful matter of fact we are using this lib to integration openstackid.org with swift object storage using app credentials
https://docs.openstack.org/keystone/latest/user/application_credentials.html
here is the PR #336

regards

[k0ka]

Released in https://github.com/php-opencloud/openstack/releases/tag/v3.5.0