Unnecessary delegate/closure allocations in UIElementAutomationPeer

_Note that I will submit a PR in the coming days for this issue, so its more of an FYI._

### Description

`GetChildrenCore` and `GetRootAutomationPeer` both allocate a new delegate and closure each time they execute with the call to `iterate` function that the callback is passed into. Given that both capture a single local parameter, we can easily just modify the function signature and pass the parameter by `ref`, saving easily 220 MB in this case.

### Showcase


### Repro

Note: This repro is specifically crafted to highlight an issue like this.

https://github.com/h3xds1nz/UIAPerformancePitfalls