Add clang-format

[tom91136]

We probably need a clang-format file to keep all the models consistent, as right now it's hand formatted. We can use https://clang-format-configurator.site/ to generate one.
@tomdeakin what do you think?

[gonzalobg]

This would be really appreciated.

[bernhardmgruber]

This would be really appreciated.

Yes, very much appreciated!

[tom91136]

Let's do this as part of the last PR before 6.0, as we got a lot of things in flight and breaking the diff is annoying.