Install and register policy files via ament_index

[mjcarroll]

It would be convenient to have CMake infrastructure for installing SROS policy files for a given package that could then be discovered by other packages for use as part of their security policy.

I would envision that this would include installing the policy files to some known location (probably the package share directory), and then registering those policies with the ament_index.

[ruffsl]

@mjcarroll , did you have a branch with the working example you showed me at snapcraft pushed anyware? @kyrofa has been looking for examples of registering files via ament_index and <export>.