Automanage firewall indices

[lkubb]

Currently, it's a very error-prone and manual process to ensure firewall rules are applied in a specific order (unless I'm missing something – always happy to learn :)).

This PR a) allows the rule index to not be specified and attempts to autodiscover an unoccupied one b) introduces a new resource (unifi_firewall_ruleset) that manages the ordering of firewall rules.

I'm not sure if this is the best approach, but I did not think a substantial change to how firewall rules are managed currently would be worth the trouble.

This resource is virtual and will be auto-imported during creation. There can only be a single resource per site + ruleset combination. It requires all rule IDs that are present in the ruleset to be specified and disallows managing rule indices if it discovers unknown rules.

Fixes: https://github.com/paultyng/terraform-provider-unifi/issues/361

This requires https://github.com/paultyng/go-unifi/pull/127, I will need to update this PR when that one is released.

[lkubb]

@paultyng @joshuaspence Any chance of getting a new go-unifi release and this provider being updated to be able to use it anytime soon? This PR relies on changes that were merged in June, but are still unreleased.