Add Postman API specification mode to API fuzzing CI configuration page

What does this MR do?

Adds Postman to the API specification modes available on the API fuzzing CI configuration. Choosing the Postman collection option from the Scan mode dropdown and filling in the API specification input will result in the FUZZAPI_POSTMAN_COLLECTION variable being filled out in the code snippet provided after submitting the form. These changes can be found in: !55601 (6b595aec)

This MR also moves the related backend classes into the AppSec namespace and appropriate sub-namespaces per the recently published guidelines. These changes can be found in !55601 (ba1adf88)

Note: this work does not need a changelog since it is under a feature flag. The issue for the feature flag rollout is #299465 (closed)

Screenshots

Postman collection selected

Before	After

Issue

This is a follow-up to #299234 (closed). See #299234 (comment 519837582) for more details