Add minimal CMake setup

[theblackunknown]

Description

This pull request introduces a minimal CMake-based build system for the openpbr-bsdf project.
It exposes one CMake target for the OpenPBR header sources, and another to build and run the minimal example.
As a convenience for clients that prefer it, we also generate installation rules to install headers and a CMake Config package.

Motivation and Context

We want openpbr-bsdf to be easily consume by other projects, and providing a CMake build system is one step in this direction as its use is pervasive in the ecosystem.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.