Make git tag and repository optional

[KyleFromNVIDIA]

Description

To allow the removal of nvcomp 2.2.0, we make the git tag and repo arguments optional, since all nvcomp versions now use the proprietary binary instead.

Fixes: #702

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• 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

[bdice]

@KyleFromNVIDIA CI is failing:

CMake Error at /__w/rapids-cmake/rapids-cmake/build/cpm/cpm_nvcomp-override-clears-proprietary_binary-makefile-build/_deps/rapids-cmake-src/rapids-cmake/cpm/detail/package_details.cmake:78 (message):
  rapids_cmake can't parse 'nvcomp' json entry, it is missing a `git_tag`
  entry

I started with a different approach, which skipped the validation of git_tag and git_url if a key proprietary_binary existed. It was basically like this:

    rapids_cpm_json_get_value(version)
    rapids_cpm_json_get_value(git_url)
    rapids_cpm_json_get_value(git_tag)
    rapids_cpm_json_get_value(proprietary_binary)

    foreach(var IN ITEMS version)
      if(NOT ${var})
        message(FATAL_ERROR "rapids_cmake can't parse '${package_name}' json entry, it is missing a `${var}` entry"
        )
      endif()
    endforeach()
    foreach(var IN ITEMS git_url git_tag)
      if(NOT ${var} AND NOT proprietary_binary)
        message(FATAL_ERROR "rapids_cmake can't parse '${package_name}' json entry, it is missing a `${var}` entry and no `proprietary_binary` was provided"
        )
      endif()
    endforeach()

However, that also failed because of something with export rules not being able to find the dependencies without a git_url / git_tag? I got stuck and then you opened this PR, so I didn't pursue it further.

[bdice]

An alternative approach proposed by @robertmaynard is in #704. I think we can close this if the other approach is acceptable.