Clang Format Configuration, master branch (2022.11.28.)

Added a default (Clang) format for C++ source (and header) files. This was triggered by the discussion on !58765 (merged).

The proposal is based on the Google C++ Style Guilde, which all of the ACTS configurations are also based on.

I tried to make the configuration as lean as possible. Only keeping "overrides" over the Google style that make the code more in line with https://twiki.cern.ch/twiki/bin/view/AtlasComputing/CodingGuidelines. Though I can't imagine that I would not have overlooked a few things.

This is mainly intended as a conversation starter. But just to point out, just adding this one file to the repository makes it possible to re-format any (C++) source file in the repository with VS Code by just pressing [Ctrl]-[Shift]-I.

Pinging @christos, @averbyts, @tadej.

.clang-format

+# Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
+
+BasedOnStyle: Google
+Language: Cpp
+AllowShortBlocksOnASingleLine: false
+AllowShortCaseLabelsOnASingleLine: false
+AllowShortFunctionsOnASingleLine: Inline
+AllowShortIfStatementsOnASingleLine: false
+AllowShortLoopsOnASingleLine: false
+KeepEmptyLinesAtTheStartOfBlocks: true