refactor: create OS Guard yaml generators (#14504)

We will have multiple flavors of OS Guard, each with minor tweaks. To
easily support this, define a common base OS Guard yaml template and
a delta yaml template that does minor adjustments to the base template.

Additionally introduce a python script to merge arbitrary image
customizer yamls together. This python script also adds a header to the
final output yaml to indicate the file was autogenerated and provides
the input source files.

Finally, introduce a bash script to specifically generate the OS Guard
final yaml configs.

Signed-off-by: Chris Co <chrco@microsoft.com>

Author: christopherco