APPLE: Enable Building as Framework

[dgovil]

Description of Change(s)

This PR separates out just the framework build part of #2969 into an isolated PR, and addresses the notes from @meshula in the process.

Overview of Changes

• build_usd.py now adds a --build-apple-framework and --no-build-apple-framework to enable building frameworks. Frameworks are enabled by default for embedded OS builds.
• When building as a framework, builds are monolithic.
• Framework builds force TBB and OpenSubdiv to be statically linked. This can be revised in the future, but we discussed this previously, and decided it was the lesser of two evils given we're moving to GCD support.
• The CMake build calls out to a zsh script (zsh is guaranteed to exist on macOS) to build the framework. This is used because CMake's own native framework build support would be far more invasive a change currently.
• A new configuration option PXR_EXTRA_BINARY_RELATIVE_RESOURCES allows the resource directory to be configured relative to the binary. This avoids hardcoding patterns in the CPP files, and allows configuration in CMake. This was requested by @meshula as well.

Checklist

• I have created this PR based on the dev branch

• I have followed the coding conventions

• I have added unit tests that exercise this functionality (Reference:
  testing guidelines)

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement (Reference:
  Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-11201

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[tlewis]

Hi @dgovil - I'm curious what the motivation is for this. It's currently not used by anything so is this ok to remove?

[dgovil]

Yeah I think it can be removed. It was vestigial from when I spilt up the PR