add support for addons

[DavidLokison]

No description provided.

[DavidLokison]

Addons would be supported by following means:

• add_addon(addon-dir) instead of add_subdirectory(addon-dir) in the CMakeLists.txt to flag a subdirectory as an addon. It only gets added to the project if the addon is enabled
• add_addon_hook(addon-dir) where you usually would fips_deps to mark the fips dependency as addon-specific. It only gets added to the dependency list if the addon is enabled
• an Addons array in the projectinfo.json with a list of strings for the addon directories to be included. This enables the addons and provides build flags like WITH_NEBULA_ADDON_{ADDON-DIR} to mark the addon-calling code.