[kotlin] fix enum default values

Created by: kalinjul

This PR fixes #12531 (closed) and implements enums for kotlin-retrofit mostly by using the template from okhttp.

I added a enumDefaultValue field in CodeGenParameter that holds the default enum for enum types. This was necessary because defaultValue holds the value of the enum, not the enum name. This is not the the same in some cases (for example, "de-DE" is converted to enum "deMinusDe"). defaultValue still holds the (String) value, so templates that do not support enums are not affected by this change.

I kept the per-method enum generation with Enum types named {{enumName}}_{{operationId}} that okhttp template used, although i think it's a bit clumsy.

This was tested in my project with kotlin jvm-retrofit2 and with okhttp and the json from the bug mentioned above.

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@jimschubert: mentioning you for review