Allow for cpm default and override files via variables

[robertmaynard]

Description

Extends https://github.com/rapidsai/rapids-cmake/pull/595 with support for setting the default and override file used by rapids-cmake via the command line.

Fixes #552 #587

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.

[robertmaynard]

/merge