Add support for mDNS on network resource

[slok]

Hi!

First of all, I want to thank you for creating this awesome provider... In combination with Terraform cloud, my home network has gone to the next level 🚀. Many thanks :)

On Controller v7, mDNS was removed (#241) as a global setting and added into each of the networks independently.

This PR adds the setting with a note that it's only supported on controllers >=v7.

Note: At first I added unit tests, but it started to get complex because this setting is not supported in all controller versions, also testing every single setting would make the tests hard to read and maintain, so I didn't add a test just for this small setting. If you think it's worth it, I can add the unit test.

[paultyng]

You can add a test but then make it skip if you aren't on the correct version, see a similar case here:

terraform-provider-unifi/internal/provider/resource_wlan_test.go

Line 304 in d61a34d

preCheckMinVersion(t, controllerVersionWPA3)

It doesn't need to cover every permutation but for regression purposes because controller versions and API change rapidly its nice to have it covered. I have this setup to run the tests on a schedule so it will continuously test against newer versions of the API so we can hopefully get out ahead of any future breaking change when possible.

[slok]

@paultyng Test added! 🚀

[slok]

Hi @paultyng!

The PR is ready, just a reminder in case it was forgotten, whenever you have time, please give it a review, many thanks!

[alexstophel]

👍 For this feature. It's essential for HomeKit to work properly and is the only part of my current network I can't provision with Terraform. Would love to see this merged and added.

[joshuaspence]

I merged a different PR which changes the function signature of getTestVLAN. But for some reason I am having trouble updating this PR so I will just merge this PR as is and fix it separately.

[slok]

Thanks @joshuaspence!