build: Add the usual Git hooks

* CMakeLists.txt: Include G10GitHooks. Call g10_configure_git_hooks.
* cmake/modules/G10GitHooks.cmake: New.

This adds the usual Git hooks used by g10 Code projects.

The file G10GitHooks.cmake is based on KDEGitCommitHooks.cmake from
KDE's Extra CMake Modules.

• GnuPG-bug-id: T7262