Enhancement: surface parse errors more comprehensively throughout the AF

[davidjwbbc]

Description

When the OpenAPI code templates were updated we introduced an "out" parameter which returns the parse error reason string. Where the OpenAPI model interfaces are directly used by the M1/M5 API this reason is now reported, but where the parsing is buried within other functions (dynamic-policy, network-assistance-session, etc.) this error reason is not surfaced back to the API response.

Design

Add a const char **parse_err parameter to all functions which perform any parsing or validation of JSON to return any parse error reason strings back up to the API handling so that it can be returned as part of the ProblemDetails description.