Add new variable _PER_REQUEST_SCRIPT
## Problem
Some forms of API authentication combine portions of the HTTP request with
a secret. Recently we have had several customers request this feature, a
workaround exists using the legacy configuration file and a deprecated
version of Python 2.7. This is not optimal as customers must manually upgrade
the configuration file if changes are made, such as adding a new check.
## Proposal
Add a new variable `DAST_API_PER_REQUEST_SCRIPT` allowing a user to provide the filename for a script. The contents of the script will be merged into the configuration file provided by `worker-entry` to API Security.
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