feat(cli): support creating draft in "gas draft" command

tensorbay/cli/draft.py

@@ -11,19 +11,24 @@
 import click
 
 from .tbrn import TBRN, TBRNType
+from .utility import get_dataset_client, get_gas
 
 
 def _implement_draft(
     obj: Dict[str, str], tbrn: str, title: str, is_list: bool  # pylint: disable=unused-argument
 ) -> None:
+    gas = get_gas(**obj)
     info = TBRN(tbrn=tbrn)
+    dataset_client = get_dataset_client(gas, info)
 
     if info.type != TBRNType.DATASET:
         click.echo(f'To operate a draft, "{info}" must be a dataset', err=True)
         sys.exit(1)
 
     if title:
-        pass
+        # todo: create draft base on revision
+        dataset_client.create_draft(title=title)
+        click.echo(f"Draft {dataset_client.status.draft_number} is created successfully")
 
     if is_list:
         pass