This PR introduces @ObsoleteApi. Deprecating any client side feature including adding @Deprecated annotation is considered a breaking change based on the new support policy, hence we need an alternative way to signal us and customers that an API is deprecating. This annotation is not a direct replacement of @Deprecated, it's more like a pre-deprecation annotation, it has two major purposes:

1. It provides a signal to us regarding what we are going to deprecate in a major version release. We can easily change all @ObsoleteApi methods to @Deprecated before a major version release, and eventually remove it in another major version release.
2. It provides a signal to customers that methods marked with it will be deprecated in a major version release, so they can start using alternative ways as soon as possible. It is not as obvious as @Deprecated since IDE would strike through deprecated methods, but customers can see it easily through Javadocs while hovering over on it.