Context in its own package?

Although `io.grpc.Context` is shipped in its own artifact, and non-gRPC users can also depend on it, the package name still causes branding issue (google/instrumentation-java/issues/162), and a practical issue with OSGI (#2727). We may consider moving it into its own package and splitting it completely from gRPC.

We have two questions to answer:
1. What should the new package name be? Probably something starting with `com.google`, but probably not `com.google.common.context` because it collides with a Google's proprietary context class.
2. How to bridge the old `Context` and `Deadline` to the new ones.

Start by reviewing the io.grpc.Context and Deadline APIs and the issue's two open questions about package naming and compatibility. No source file or test is named, so first identify where these classes and their consumers are defined. Done requires an agreed package split and a clear bridge for existing Context and Deadline users.

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