Improvements to subsystem query API

[Zee2]

Overview

These are some significant improvements to the subsystem query API that were originally part of the new_hand_menu branch/PR. I still have a few last things to add to that before merging, but I wanted to get these improvements in ASAP.

Changes

• Adds handy conversion operator to HandJointPose to automatically cast it to a Pose
• Less confusing doccomments on Handedness
• Removed confusing doccomment remark on XRSubsystemUtilities
• Refactors XRSubsystemHelpers to internally reuse scratchpad List<T>s, cached by generic type
  • This does mean that the list reference will be reused/squashed for subsequent calls, so calling clients shouldn't hold on to a reference to the list for longer than necessary. But for the vast majority of instances where people are trying to get a reference to a single subsystem, this is a huge improvement (i.e., just trying to access the hands API)

[Zee2]

Profiling results here. Takeaways: the auto-scratchpad-list-system is equally performant to maintaining your own reusable list yourself. The .HandsAggregator property is very very cheap (0.14ms for 10,000 calls per frame). Allocing every query is very inefficient, as expected. The .running check is free.

Overall, the auto-scratch-list is 2x efficient over the naive call, and the direct helper property is an 86.7x speedup over the naive call.

// Just checking .running property
// 0.00ms, GC = 0
bool test = true;
for (int i = 0; i < 10000; i++)
{
    test &= hands.running;
}

// Direct helper property, cached
// 0.14ms, GC = 0
for (int i = 0; i < 10000; i++)
{
    hands = XRSubsystemHelpers.HandsAggregator;
}

// Auto-scratch-list
// 6ms, GC = 0
for (int i = 0; i < 10000; i++)
{
    hands = XRSubsystemHelpers.GetFirstRunningSubsystem<HandsAggregatorSubsystem>();
}

// Using Unity API directly, no reused list
// 8-12ms, GC = 20,000 (1.0 MB)
for (int i = 0; i < 10000; i++)
{
    var list = new List<HandsAggregatorSubsystem>();
    SubsystemManager.GetSubsystems(list);
    if (list.Count > 0)
    {
        hands = list[0];
    }
}

// Using Unity API directly, reusing list
// 6ish ms, GC = 0
for (int i = 0; i < 10000; i++)
{
    SubsystemManager.GetSubsystems(reusableList);
    if (reusableList.Count > 0)
    {
        hands = reusableList[0];
    }
}

[keveleigh]

Missing a null check on the aggregator here and in GrabPosition below

[Zee2]

Will do in another PR so that the change won't reset the approval :D

[david-c-kline]

Overall looks good, there are a couple of questions and it would be good to resolve the null check issues that @keveleigh pointed out.