This largely copies over Spicy's `.clang-format` configuration file. The
one place where we deviate is header include order since Zeek depends on
headers being included in a certain order.
This adds two example plugins within testing/builtin-plugins/Files:
* protocol-plugin copied over from testing/btest/plugins/protocol-plugin
* py-lib-plugin that embeds Python to have a dependency on an external
shared library which was already available in CI and fun to use, too.
Closes#2837