APPLE: Make codesigning improvements

[dgovil]

Description of Change(s)

This PR overhauls the current code signing setup with the following changes:

1. Developers can now pass in code signing identifiers directly to the build_usd.py script.
2. Codesigning identifiers are now passed to CMake for Xcode targets if they want to sign anything.
3. The code signing identifier is calculated less often
4. Codesigning now supports signing frameworks
5. Codesigning supports checking previous signing status and only re-signing as needed. Note: Since USD's cmake always installs all targets again, they're always re-signed. Future work might help optimize this, but at the least we're not re-signing every dependency.
6. Codesigning is validated earlier to prevent running through a whole build just to find code signing fails

This makes code signing much more reliable and faster overall.

This work is needed to enable properly building OpenUSD as a framework

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-11198

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[sunyab]

Hi @dgovil , left some notes from a first pass review. Thanks!

[dgovil]

Thanks, @sunyab !
I'll take a pass next week at resolving these

[dgovil]

Thanks, @sunyab
I pushed an update that also rebases on top of dev.

Appreciate the notes. Hope you have a good thanksgiving break.