Introduce `KSerializer` for enums in `Response` models

[StefMa]

See reason here: #80, specially here.

We should introduce a KSerializer for each enum that has a default value right now.
For example like this:

kmp-passenger-api/library/src/commonMain/kotlin/com/ioki/passenger/api/models/ApiRideResponse.kt

Line 87 in 6ab4344

@SerialName(value = "receipt_type") val type: Type = Type.UNSUPPORTED,

There we should remove the default value and introduce a custom KSerializer like that:

kmp-passenger-api/library/src/commonMain/kotlin/com/ioki/passenger/api/models/ApiRideResponse.kt

Line 128 in 6ab4344

private object ApiCancellationReasonSerializer : KSerializer<ApiCancellationReason?> {

TODOs:

• Search for all default values in code (for enums)
• Remove default values for enums
• Introduce custom KSerializer for those