Give the webhook handler a timeout (closes #38)

The webhook handler's client had no timeout, and slog calls the handler
inside the log call, so a server that accepted the connection and never
answered stopped that log call for good, and every later one. A request
still running after 5 seconds, reading the answer included, now fails
with a timeout error. The handler also reads the answer to the end
before closing it, so the connection is reused for the next record.

Two new tests point the handler at a server that never answers and at
one that sends its status and then stalls the answer, and check that
Handle returns a timeout error within the timeout. The README states
the timeout.

Model: opus-5-5

README.md

@@ -116,7 +116,9 @@ the record:
     stdout, wrapped, so `errors.Is` still matches the original
 -   `WebhookHandler` returns an error when the request fails, and also when
     the server answers with a status outside 2xx, a redirect included,
-    since it does not follow redirects
+    since it does not follow redirects. A request that has not finished
+    after 5 seconds fails with a timeout error, so a webhook server that
+    never answers holds up a log call for 5 seconds at most
 -   `MultiplexHandler`, which simplelog installs as the default, passes the
     record to every handler it holds even after one of them fails, then
     returns all their errors joined with `errors.Join` (nil if none failed)