APPLE: Fixes for Xcode builds and debugging

[creijon]

Description of Change(s)

This PR combines the work in the following two PRs:

#1937
#1946

In addition, Xcode builds require code signing. This has been added as a separate python file, apple_utils.py

Debugging usdview or usdrecord in Xcode requires using the python executable. However this is does not have the necessary entitlements for debugging, so Xcode will not connect. To fix this, it is necessary to disable System Integrity Protection (SIP) in the Recovery OS. You should only disable SIP temporarily to perform the debugging, and reenable it as soon as possible. Failure to reenable SIP when you are done testing leaves your computer vulnerable to malicious code.

More information and instructions on how to disable SIP can be found here:

https://developer.apple.com/documentation/security/disabling_and_enabling_system_integrity_protection

Fixes Issue(s)

• Building and debugging in Xcode

• I have submitted a signed Contributor License Agreement

[sunyab]

Filed as internal issue #USD-7517

[meshula]

This diff is getting pretty small, introducing an env var to point to the libraries for debugging, and also bumping the libpng version. Is the libpng version bump related to xcode building and debugging? If no, maybe that bump should be split out into its own PR.

[davidgyu]

We've merged the libpng version update with dev commit 63a7c17

The changes to base/lib/plug aren't needed since you can specify multiple plugin search path locations via the existing PXR_PLUGINPATH_NAME environment variable as described in BUILDING.md, e.g. "<installdir>/lib/usd:<installdir>/plugin/usd"

[tallytalwar]

Closing as per @davidgyu 's comments above. Thanks all.