build: Add "gen-swdb", "release", and "sign-release" targets

* CMakeLists.txt: Include G10AddReleaseTargets. Add the release targets.
* cmake/modules/G10AddReleaseTargets.cmake,
cmake/modules/g10_generate_swdb.sh.in,  cmake/modules/g10_release.sh.in,
cmake/modules/g10_sign-release.sh.in: New.

This adds the custom "release" and "sign-release" build targets used by
g10 Code projects. Additionally, it adds a "gen-swdb" target that's used
internally by the release target.

• GnuPG-bug-id: T7262