Apple: Core visionOS Support

[dgovil]

Description of Change(s)

This PR builds on the Core iOS support PR to add support for building the USD core for visionOS.

Thanks to Valentin Roussellet for the basis of the TBB patch.

It includes the following changes:

• Use CMake 3.28 (which adds visionOS support)
• Patch TBB to support visionOS. Note that clang requires new platforms to be provided via the target triplet.
• Add visionOS to the accepted platforms.

Note that visionOS's SDK is called xros, much like how the iOS SDK is iphoneos for historical reasons.

• [X] I have verified that all unit tests pass with the proposed changes
• [X] I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9587

[jesschimein]

/AzurePipelines run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).