chore: import upstream snapshot with attribution

docs/md/how_to/python/virtual_server/custom.md

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+# Implementing a custom Virtual Server
+
+You can connect Perspective to any data source by subclassing
+`VirtualServerHandler` and wrapping it with `VirtualServer`.
+
+For background on virtual servers, see the
+[Virtual Servers overview](../../../explanation/virtual_servers.md).
+
+## Example
+
+```python
+from perspective import VirtualServerHandler, VirtualServer
+
+class MyModel(VirtualServerHandler):
+    def get_features(self):
+        return {
+            "group_by": True,
+            "split_by": False,
+            "sort": True,
+            "filter_ops": {
+                "string": ["==", "!=", "contains"],
+                "float": ["==", "!=", ">", "<"],
+            },
+            "aggregates": {
+                "float": ["sum", "avg", "count"],
+                "string": ["count"],
+            },
+        }
+
+    def get_hosted_tables(self):
+        return ["my_table"]
+
+    def table_schema(self, table_name):
+        return {"name": "string", "price": "float"}
+
+    def table_size(self, table_name):
+        return 1000
+
+    def table_make_view(self, table_name, view_id, config):
+        # Translate `config` (group_by, sort, filter, etc.) into a
+        # query against your data source. Store the query keyed by
+        # `view_id` for later data retrieval.
+        pass
+
+    def view_delete(self, view_id):
+        # Clean up resources for this view
+        pass
+
+    def view_get_data(self, view_id, start_row, end_row, start_col, end_col, ctx):
+        # Execute the stored query with the given row/column window.
+        # Push results via `ctx`.
+        pass
+```
+
+The `VirtualServer` instance can then be passed to a Tornado, Starlette, or
+AIOHTTP handler just like a regular `Server`:
+
+```python
+from perspective.handlers.tornado import PerspectiveTornadoHandler
+
+app = tornado.web.Application([
+    (r"/websocket", PerspectiveTornadoHandler, {"perspective_server": VirtualServer(MyModel)}),
+])
+```