Fix iOS Resolver to produce :tvos target podfiles

[DellaBitta]

Updates to IOSResolver.cc to handle reading of UnityEditor.PlayerSettings.tvOS.targetOSVersionString for when active build targets are tvOS. Previously the UnityEditor.PlayerSettings.iOS.targetOSVersionString was used.

This work involved:

• A general breakout of TargetSDK into IosTargetSDK and tvOSTargetSDK.
• Renamed TargetSDKVersion to TargetIosSDKVersionString to better differentiate it from TargetIosSDKVersionNum, and did likewise for the new tvOS counterparts.
• Added a new poller to monitor changes to PlayerSettings.tvOS.targetOSVersionString akin to what we do for the PlayerSettings version value on iOS.
• In GenPodfile(), when the active build target is tvOS, populate the Podfile with platform target :tvos instead of platform target :ios and use the version string from PlayerSettings.tvOS instead of PlayerSettings.iOS.

[DellaBitta]

We spoke offline. The Polling mechanism for the targetSDK values requires that TargetIosSDK and TargetTvosSDK be class properties. Condensing the TargetSDK logic into a single method would probably create even more code than reduce it, as it would simply replicate the code currently in the properties but in a new location with switch/case statements, etc.

So merging this as is for now!