[FEATURE] allow grant type to be specified for client:auth

[sandragolden]

closes #245

I do not see this as a breaking change because everything will continue to work normally but we have a new option that will allow you to force client_credentials grant type

Related to Resource Owner Password Credentials Authentication Failure

What PR adjusts: Allows grant type to be specified for client:auth. For example: sfcc-ci client:auth -t client_credentials. Default grant type will remain unchanged and is set to password by default.

Why? I'm running a local project where I'm running into the Error: Authentication failed: Resource owner authentication failed mentioned in the related doc above. Because I have a dw.json file in my project, the grant type of password is always being used. Due to my Account Manager setup, this is throwing an error. If I force the grant type of client_credentials, everything works as expected because I've added the necessary permissions to my client.