[FR]: Add support of Zephyr OS

[anobli]

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Does the feature exist in the most recent commit?

This doesn't exist.

Why do we need this feature?

Zephyr has its own test suite (ztest) but it has a limited support of C++. Supporting Zephyr OS would simplify C++ testing for Zephyr application written in C++ and would allow people already familiar using googletest keep using it with Zephyr OS.

Describe the proposal.

I would like updating the sources to support Zephyr OS. In addition, to facilitate its integration in Zephyr OS, I would also like adding some additional files that allows Zephyr build system to download googletest and build it properly for zephyr.

Is the feature specific to an operating system, compiler, or build system version?

Specific to Zephyr OS