Cleanups 🧹

[bignimbus]

This is a list of changes that we don't want to forget about. They are likely to be backwards-incompatible so most of these will need to wait for a future major release. This list is mostly a braindump and is missing a lot of context. This is not about new features but rather a list of small cleanups to do.

Continued from #4171

If you see anything that doesn't make any sense, it's probably because it doesn't. Feel free to comment and we'll dive into it.

Potential Breaking Changes/cleanups

• Enforce opt-in the kotlin_labs linking
• Rename Foo.type to Foo.__type to avoid nameclashes (or just leave it like this because we can detect the case. See 'type' is a reserved enum value name #4293)
• Rewrite the buffered adapters to not use JsonReader anymore
• Cleanup the JsonReader API and distinguish between the buffer/non buffered case.
• Generate the customScalarAdapters?
• Make the field merging validation more strict (see Add a flag to disable fieldsCanMerge validation on disjoint types #4342 (review))
• Use scalar instead of customScalar API cleanup: use "scalar" instead of "custom scalar" #3815
• Refactor shapes()
• Move httpInfo and others outside of executionContext
• Improve the Java ergonomics of HttpKt
• (To be considered): pass the variant name to the lambda for createAllAndroidVariantServices (see this comment) (or deprecate the function?)
• Rename WsProtocol.Factory to WsProtocol.Builder
• Remove IdlingResource. See this comment from OkHttp for instance, and this article about how to avoid them with Compose.
• Remove circular dependencies between Gradle projects
• extract HttpEngine and WebSocketEngine to a common module
• Remove Gradle defaultService