Expose `AbstractRestProtocol` class outside

[timocov]

Currently the class has a javadoc saying

This class is currently package-private, but may be made public in the future when we're sure about its API.

And it has been 7 years since introducing this class, do you have any plans to expose it to the public? 🙂

[kstich]

We currently have only the one implementation that uses it (AwsRestJson1Protocol), meaning we're not sure we've got the abstraction 100% correct.

We could open it up with @SmithyUnstableApi, but that's still some risk to accept.