webhook: Add initial webhook plugin (#39)

This initial plugin implementation is very simple. It does a bunch of
simplifying things:
- Assumes `POST`.
- Assumes JSON request and response bodies.
- `body` and return are `Any`, so we won't get much typing help.

This is good enough to be used by a workflow with the description:
> Post a body of the form { "message": "Hello from Lutra and Zapier!"}
to the webhook `https://hooks.zapier.com/hooks/catch/<elided>`. Allow
only the message to be customizable.

webhook/plugin.py

@@ -0,0 +1,14 @@
+from typing import Any
+
+import httpx
+
+
+async def webhook_request(url: str, body: Any) -> Any:
+    """
+    Make a POST request to the given URL with the given body and returns the response.
+
+    Both the request and response bodies are expected to be JSON and are converted to
+    and from Python values using the json module.
+    """
+    async with httpx.AsyncClient() as client:
+        return (await client.post(url, json=body)).raise_for_status().json()

webhook/plugin.toml

@@ -0,0 +1,2 @@
+name = "Webhook"
+description = "Use external webhooks."