[Proposal] Introduce clang-format

[itrofimow]

Hi!

Basically, title: recently I'm reading a lot of C++ code of Vespa and also writing some,
and all the different naming conventions/formatting styles there are in the codebase
are a bit confusing to deal with.

What do you think about introducing a common .clang-format to format a new code with,
and gradually going from there?

[itrofimow]

To clarify, I don't have any specific config in mind (I have my preferences, but no strong opinions here),
but I think that anything ubiquitous could be of benefit