Add support for Gateway's

[NOXITB]

Add Support For gateways and APS

[charlie-haley]

AP's are already supported, they're treated as device metrics, e.g omada_device_cpu_percentage. Were there any AP metrics you're missing and would like adding?

I also don't have use an Omada Gateway so I'm unable to test it out, more than happy to accept contributions for this feature

[NOXITB]

@charlie-haley Sorry let me clarify the gateways dont have ports displayed in the metrics

[charlie-haley]

It's most likely down to the port metrics being a different property on the Gateways, as I dont own the Omada gateway this is pretty difficult to test.

I'm hoping the Open API work will enable making this easier, along with some automated tests #81