[Enhancement] Ability to specify directory with file regex, such as for cdk.out

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

When using cdk, the template files have the suffix "*.template.json". Looking to run cfn-guard for only the cloudformation templates in the cdk.out directory and not all files within the given directory.

**Describe the solution you'd like**

Either a cdk specific toggle or a flag to support file regex such as "*.template.json" or "projectname*.template.json"

**Describe alternatives you've considered**

Would alternatively need to find the templates and then iteratively invoke cfn-guard

**Additional context**

Add any other context or screenshots about the feature request here.

The issue does not name a source file or test. Start by locating cfn-guard's directory input handling and CLI argument parsing, then determine how a directory such as cdk.out can select matching *.template.json files. Done means the requested file pattern or equivalent cdk-specific selection works without invoking cfn-guard separately for each template.

Written by the indexing model from the issue text.