Add policy id

[FH-Inway]

When handling errors in policies, it can be useful to identify policies via their id attribute.

I have implemented a proof of concept for set-variable in FH-Inway@21d405d.
However, I wonder if there is a better way to implement this optional argument that I believe would apply to all policies. Right now, to fully support this, it would require a change in all policy statements in all the context interfaces and in all the policy compilers.