fix(generator): OpenAPI generator issues with Petstore example

[chillleader]

Description

1. Added a "target" prefix to all property IDs so they look like operationId_body_propertyName or operationId_headers_propertyName. Sometimes properties in body (derived from the JSON schema) can have the same name as parameters.
2. Fixed an issue with scan command where unsupported operations were never displayed
3. Changed the generate command output so it returns a single JSON object when only one template was generated, instead of an array of element template JSON objects.
4. Adjusted the README: removed the invalid example, added a note about unsupported operations.

Related issues

See https://camunda.slack.com/archives/C0350959WG1/p1705053321728369

[sbuettner]

Great improvements.