Preserve allowed_regexp_prefixes repo level key

[taylor-fis]

Feature Request

• I'd be willing to implement this feature

Describe the idea
Support preserving the allowed_regexp_prefixes key. Currently this key does not get preserved between runs of terragrunt-atlantis-config.

Why is this helpful?
Allows us to restrict what regexes can be used to multi plan/apply terragrunt modules.

What alternatives exist today?
The current workaround is to generate the atlantis config and then use yq to merge the files which is not ideal.

terragrunt-atlantis-config generate --create-project-name --depends-on --parallel=false --workflow terragrunt --autoplan --output /dev/stdout | yq eval-all --inplace 'select(fileIndex == 0) * select(fileIndex == 1)' atlantis.yaml -