[Kotlin][Multiplatform] Declare API classes and methods as open

Created by: tkirshboim

This PR changes the Kotlin multiplatform templates so that:

• API classes can be subclassed.
• API methods can be overridden.

The motivation for this change is to make it easier to use API classes in tests. By making the class and methods open, it is possible to override the API class behavior and provide stubs.

An example of how this is useful in a test scenario can be seen here.

private class PetApiStub(private val findPetsByStatusResponse: List<Pet>) : PetApi() {
    override suspend fun findPetsByStatus(status: List<String>): HttpResponse<List<Pet>> =
        HttpResponseStub(responseStub = findPetsByStatusResponse)
}

CCs: Kotlin technical committee: @jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m

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