PY-207 nq.runs, nq.Filter, nq.Attribute, nq.AttributeFilter

I want to make working with nq easier by allowing to import runs,
Filter, Attribute and AttributeFilter straight from `neptune_query`
(often aliased as `nq`):

```
import neptune_query as nq
nq.runs.list_attributes(
  runs=nq.Filter.eq(nq.Attribute("sys/name"), "my-run"),
  attributes=nq.AttributeFilter(name="abc/"),
)
```

Author: gabrys

src/neptune_query/__init__.py

@@ -15,14 +15,19 @@
 #
 
 __all__ = [
-    "set_api_token",
-    "list_experiments",
-    "list_attributes",
+    "download_files",
     "fetch_experiments_table",
-    "fetch_metrics",
     "fetch_metric_buckets",
+    "fetch_metrics",
     "fetch_series",
-    "download_files",
+    "list_attributes",
+    "list_experiments",
+    "set_api_token",
+    # Convenience exports (see the bottom of the file)
+    "Attribute",
+    "AttributeFilter",
+    "Filter",
+    "runs",
 ]
 
 import pathlib
@@ -499,3 +504,17 @@ def fetch_metric_buckets(
         include_point_previews=include_point_previews,
         container_type=_search.ContainerType.EXPERIMENT,
     )
+
+
+# PY-207 Convenience exports for things like:
+#
+# import neptune_query as nq
+# nq.runs.list_attributes(
+#   runs=nq.Filter.eq(nq.Attribute("sys/name"), "my-run"),
+#   attributes=nq.AttributeFilter(name="abc/"),
+# )
+#
+# Keep at the end of the file to avoid circular dependencies
+
+from neptune_query import runs
+from neptune_query.filters import Attribute, AttributeFilter, Filter