Format cpp,h,C files

[Victor-Schwan]

BEGINRELEASENOTES

• Add a .clang-format file using the LLVM style
• Format all .cpp, .C and .h files
• Add formatting commit hashes to .git-blame-ignore-revs

ENDRELEASENOTES

[jmcarcell]

Why separate formatting macros and files? Can you putt all formatting in the same commit?
For the .clang-format did you pick the Key4hep one? We never decided anything for iLCSoft, but it would make it somewhat consistent (even if the format that has been used in the past in iLCSoft repositories is different). If one day we decide to have a common .clang-format in iLCSoft repositories then no extra formatting would be needed.

[Victor-Schwan]

At first, I wanted to format the macros and only then I realized that I probably should format every relevant file in the repository. Ofc I can combine the commits.
I have used the LLVM style but I am fine with using which ever style you prefer. Assuming that the key4hep style is consistent in all repos, I would just copy the one from key4hep/k4FWCore, ok?