Review design of POST /call-forwardings response body and consider changing

[eric-murray]

Problem description
The response body is defined as an array which can contain between 1 and 5 entries from the set "inactive", "unconditional", "conditional_busy", "conditional_not_reachable" and "conditional_no_answer".

The issues with this design are:

• The possible values are not independent. If "inactive" is present, no other value should be present.
• Given that "inactive" is mutually exclusive with the other values, it is not possible to have 5 valid items in the response array without duplicated values

Expected behaviour
Consider redesigning the response body as follows:

• Remove "inactive" as a valid response value, instead representing this by the empty set [] (i.e. no call-forwardings active)
• Set minItems to 0 and maxItems to 4
• Enforce that array values cannot be duplicated by specifying uniqueItems: true

Alternative solution
None proposed

Additional context
None