Allow setting/updating the logfile field of Scalyr endpoints

[cee-dub]

Is your feature request related to a problem? Please describe. There's a UI feature to change the logfile name sent to Scalyr, but it isn't exposed in the CLI.

Describe the solution you'd like I have opened a PR against https://github.com/fastly/go-fastly to expose the API field used by the UI. The CLI needs to be updated to match.

Describe alternatives you've considered Not using this field. Having my app log its own field instead.

Additional context