Remove "agrest-client"

[andrus]

As a part of stack refactoring, we are going to simplify our modules architecture by removing "agrest-client". "agrest-client" hasn't been supported for years and is falling behind. And Agrest can be queried directly via HTTP, and a special client doesn't provide much value.

The most valuable (and less trivial) piece of the client is the ability to read the data response as ClientDataResponse<T>, with T objects being actually created. We might eventually provide a utility method in Agrest to achieve that without the client runtime.

Would have been great if server-side DataResponse could have been easy to deserialize from the client, but since it contains encoders and does not contain the total, it is not as easy to use as a POJO.