Introduce clang-format to rapids-cmake to format C++ code examples

[robertmaynard]

Description

In preparation of more C++ code being added to rapids-cmake introduce a .clang-format file to the project to keep everything with a consistent style and formatting.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.

[vyasr]

How did you choose the settings? Are they copied from another project?

I was going to recommend adding a pre-commit hook, but it looks like we've already got one. Have we been running it without a format file?

[robertmaynard]

I copied the format file from cudf.

Yeah we have been running format without any file so it was using the defaults. But I don't think we ever ran it over the existing c++ files

[bdice]

Thanks! This simplifies #328.

[bdice]

/merge