hdEmbree: add support for building against embree4

[mattyjams]

Hello!

Description of Change(s)

These changes allow building USD and the hdEmbree plugin against the recently released 4.0.0 version of Embree. The upgrade from 3.x to 4.x of Embree involves a filename change of headers and libraries (embree3 -> embree4), the renaming of the header file that contains the version information from rtcore_version.h to rtcore_config.h, and for the usage in hdEmbree, the removal of the RTCIntersectContext type.

The last change in https://github.com/mattyjams/USD/commit/98b171aa83f27fa4cdb6ab74d241c1d1727e27ea isn't strictly necessary and may be overkill since there isn't really a precedent for being able to choose versions of dependencies from the command-line, so I wouldn't mind if that change needed to be culled. It does provide an easy way to select Embree 4 rather than Embree 3 though, so I included it in case it might be useful for others.

Many of the imaging tests still appear to be disabled by default on Windows which is the platform I'm currently on, so I have not been able to verify them, though the rest of the test suite passes. Manual testing in usdview has been successful so far as well.

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

• [ X ] I have submitted a signed Contributor License Agreement