Include CPM.cmake within SoCMake

[Risto97]

It is a bit annoying having to add CPM.cmake file to every IP block repository.
It would be possible to include the CPM.cmake within SoCMake.

This way SoCMake can be fetched with FetchContent, and then from there CPM.cmake would be available for other repositories:

Something like this:

FetchContent_Declare(SoCMake
  GIT_REPOSITORY  https://github.com/HEP-SoC/SoCMake.git
  GIT_TAG develop
  # OPTIONS DOWNLOAD_CPM It would be nice if it can be an option TODO investigate
)
FetchContent_MakeAvailable(SoCMake)

# After this `CPM.cmake` should be available from now on

CPMAddPackage("gh:fmtlib/fmt#7.1.3")