feat: support /functions endpoint in cloud

[dgnorton]

Add CLI support for the /functions endpoint in Cloud:

https://github.com/influxdata/openapi/blob/master/contracts/svc/managed-functions.yml#L8

If attempting to use this feature with an OSS instance, the CLI should report a Supported in cloud only or similar error.

This functionality is, currently, partially implemented by PR https://github.com/influxdata/influx-cli/issues/153.

As part of this ticket, the functions flags should be un-hidden in the auth create command. See #325 for the PR that added the flags as hidden.