Time out every HTTP connection instead of waiting forever

No ureq agent set a timeout, and ureq sets none by default, so a connection
dropped silently mid-transfer (a NAT or load-balancer idle drop) hung a JMAP,
DAV, EWS or Graph run, or an export, with no error, and the retry logic never
got a chance to run. IMAP and ManageSieve already had read timeouts.

Every agent now takes its settings from a new net module: 30s to connect,
60s to send the request headers, 5 minutes for the server's first byte, and
30 minutes for a whole response body, which ureq counts as one budget for the
body rather than per read: enough for the 512 MiB limit at about 300 KB/s.
A JMAP upload's send budget grows with its size, from a 2 minute floor at an
assumed 64 KiB/s worst case.

A timeout is a transport error, and every client already retries those, so a
stalled transfer is now abandoned and retried. Tests pin that down for each
client. The TLS setup the seven agents repeated moves to one helper.

src/lib.rs

@@ -1,5 +1,6 @@
 /*
  * SPDX-FileCopyrightText: 2020 Stalwart Labs LLC <[email protected]>
+ * SPDX-FileCopyrightText: 2026 John Coffey <[email protected]>
  *
  * SPDX-License-Identifier: Apache-2.0 OR MIT
  */
@@ -16,6 +17,7 @@ pub mod inspect;
 pub mod jmap;
 pub mod logging;
 pub mod managesieve;
+pub mod net;
 pub mod secret;
 pub mod sync;
 pub mod types;