[Enhancement] Support including/import a guard file from another guard file.

**Is your feature request related to a problem? Please describe.**

I currently have variables defined in cfn-guard which I would like to be able to import into multiple guard files. These variables contain list of AWS Account, OrgId, OrgPaths and there are also common guard functions which I would like to share across multiple guard files.

**Describe the solution you'd like**

One of the two options:
1) Ability to include a guard file
2) Ability to define global scope variables and functions. Then I could simply invoke a list of guard files and the ones with global scope would be available in the context.

**Describe alternatives you've considered**

At the moment I'm considering to generate guard rules on the fly, by merging multiple files prior to the execution.

The issue names no files, tests, or entry points. Start by locating the guard-file parsing and evaluation entry points, then clarify whether file inclusion or global variables and functions is the intended design; done means shared definitions work across multiple guard files with verified behavior.

Written by the indexing model from the issue text.