feat: add cli annotation subcommand (#2539)

add `cellxgene annotate` subcommand for invoking MLflow model to generate new `obs` annotations, initially intended for cell type annotations.

test/unit/cli/mlflow_model_fixture.py

@@ -0,0 +1,20 @@
+import shutil
+from tempfile import TemporaryDirectory, mkstemp
+
+import mlflow
+
+
+def write_model(model) -> str:
+    with TemporaryDirectory() as mlflow_model_dir:
+        mlflow.pyfunc.save_model(mlflow_model_dir, python_model=model)
+        return shutil.make_archive(mkstemp()[1], "zip", mlflow_model_dir)
+
+
+class FakeModel(mlflow.pyfunc.PythonModel):
+    def __init__(self, input_to_output: dict = {}):
+        self.input_to_output = input_to_output
+
+    def predict(self, context, model_input) -> None:
+        # this stdout output is useful for validating the input in a test, noting that this model will be invoked in a
+        # subprocess, so stdout is one means of communicating information back to the test code
+        print(f"__MODEL_INPUT__={model_input.iloc[0][0]}")