refactor: SessionStateExt and SessionConfigExt (apache#1130)

* refactor: SessionStateExt and SessionConfigExt

moving them out from util to their own module as
util is too busy, also it will align with naming
in used in client.

also, split `SessionConfigExt` to `SessionConfigExt`
and `SessionConfigHelperExt`. `SessionConfigExt`
will stay user facing, `SessionConfigHelperExt`
will be used internally. This reduces ballista
client public api

* remove deprecated context creation

ballista/client/src/extension.rs

@@ -15,10 +15,11 @@
 // specific language governing permissions and limitations
 // under the License.
 
-pub use ballista_core::utils::SessionConfigExt;
+use ballista_core::extension::SessionConfigHelperExt;
+pub use ballista_core::extension::{SessionConfigExt, SessionStateExt};
 use ballista_core::{
     serde::protobuf::{scheduler_grpc_client::SchedulerGrpcClient, CreateSessionParams},
-    utils::{create_grpc_client_connection, SessionStateExt},
+    utils::create_grpc_client_connection,
 };
 use datafusion::{
     error::DataFusionError,