[Kotlin] Improve client generator functionality

Description

This is a tracking issue for adding functionality to the Kotlin client generator.

• Support "default" responses. Currently 200 OK isn't handled, for example, in PetApi#addPet. Revisit this. It appears to be an issue with the petstore.yaml used by most generators, which doesn't define a 200 or "default" response for PetApi#addPet.
• Support enum type generation (see #5769)
• Support inline/embedded enums (see #5769)
• Add model tests for top-level enums and inline/embedded enums
• Support collectionFormat (multi) (see #5792)
• Test/Implement arrays of maps, arrays, objects
• Test/Implement maps of arrays/maps, objects
• Support allOf via composition
• Support allOf via inheritance
• (optional?) Per-request modification of headers,authorization
  • NOTE: ApiClient.defaultHeaders can be modified once to globally set headers for auth, user-agent, etc.
  • Test, or better implement basic auth support
  • Test, or better implement OAuth support
  • Test, or better implement apiKey auth
• Generate Api tests
• Generate Model tests
• Generate Api docs (see #5731)
• Generate Model docs (see #5731)
• Update README.md to be more project-specific rather than generic information about the generator/template (see ##5731)
• Consider an option for android targeting (--additional-properties=android=true), to use built-in serialization and other features
• Add support for Retrofit library (Ref: https://github.com/swagger-api/swagger-codegen/issues/6368)
• Add an option to select date library (Ref: #6892)

Related issues

• #5071

Review the unchecked items in the Kotlin client generator tracking list, starting with the existing PetApi#addPet response behavior and the referenced enum and collectionFormat issues. Then inspect the generator's existing API and model test coverage, including the requested tests for top-level and inline enums, arrays, maps, authentication, and generated documentation. Done means selecting a focused item and adding its implementation and tests.