[REST Connector] Support returning multiple headers

[sbuettner]

Is your feature request related to a problem? Please describe.

In some cases an upstream server might return multiple values for a single header in the Set-Cookie case for example. We should support multiple headers with the same name when they are returned from an upstream service in the REST Connector.

Describe the solution you'd like

When the REST Connector receives multiple values for the same header it should return the list of values inside the header map:

{"headers": {
  "regularHeader": "regularValue", 
  "Set-Cookie": ["1234", "mySpecialAuthToken"] 
}}

Additional context

https://jira.camunda.com/browse/SUPPORT-25027