feat: support /functions endpoint in cloud

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.

Start by tracing the CLI's existing cloud resource commands and the auth create command, then compare their request handling with the managed-functions contract at the linked /functions endpoint. Review the partial implementation referenced in #153 and the hidden flags from #325; done means /functions works in Cloud, OSS reports a cloud-only error, and the functions flags are visible in auth create.

Written by the indexing model from the issue text.