ci(cmake): split additional_commands to its own file

[JohelEGP]

I'm an user of cmake-format.

I'd like to make use of the additional_commands defined in this repository's .cmake-format.
So I setup my build to copy it to cmake/.cmake-format-additional_commands-cpm.
Unfortunately, that includes the format portion.
And it seems that cmake-format processes those last, so my own format is overwritten.

In this PR, I split the additional_commands in .cmake-format to under cmake/.cmake-format-additional_commands-cpm.
To avoid polluting the top-level, I reuse existing cmake directory.
I prefer this approach to splitting because it makes updating the resulting config file a simple CMake file(COPY_FILE) call.

Now, users of cmake-format that want their calls to CPM functions well-formatted
just need to add include: ["cmake/.cmake-format-additional_commands-cpm"] to their .cmake-format.

[TheLartians]

Thanks for the PR! Splitting the code for CPM commands makes a lot of sense.