rapids_cpm_gtest adds support for BUILD_STATIC

[robertmaynard]

Description

Extends the BUILD_STATIC option from GBench and NVBench to GTest. This will allow RAPIDS projects to package tests without any testing dependency libraries.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.
• [x] The cmake-format.json is up to date with these changes.
• [ ] I have added new files under rapids-cmake/
  • [ ] I have added include guards (include_guard(GLOBAL))
  • [ ] I have added the associated docs/ rst file and update the api.rst

[robertmaynard]

Looks good to me. Thanks! Please open an issue (rapidsai/build-planning or this repo) where we can track migration of all RAPIDS packages to use static benchmarks/tests, if one doesn't exist yet.

https://github.com/rapidsai/build-planning/issues/32

[robertmaynard]

/merge