Scope --allow-invalid-certs to the server the user named

The flag switched certificate checks off for every connection in the run.
That included the Microsoft sign-in endpoints, so a user passing it for a
self-signed source also sent refresh tokens, device codes and EWS client
secrets over unverified TLS. It also covered the export target, and any host
a server redirected to or named for its API, uploads or downloads.

It now applies only where the user pointed it: the host of --url, for the
source of an import or the target of an export. For an Exchange import with
no --url, it covers the mailbox's own domain, where on-premises Autodiscover
looks, and then only the EWS endpoint Autodiscover finds. The Microsoft and
Google sign-in and cloud hosts are always verified, with or without the flag.

Each HTTP client keeps a verifying agent and, only when the flag applies, a
second one that accepts invalid certificates, and picks per request by host.
The sign-in modules no longer take the flag at all. Autodiscover v2, which is
Microsoft's own service, is always verified.

src/jmap/request.rs

@@ -786,6 +786,7 @@ fn decode_set(mr: &MethodCall) -> SetOutcome {
 
 #[cfg(test)]
 mod tests {
+    use crate::net::CertOverride;
     use std::cell::Cell;
 
     use super::*;
@@ -798,7 +799,7 @@ mod tests {
                 token: "t".to_owned(),
             },
             RetryPolicy::new(max_retries),
-            false,
+            CertOverride::none(),
         )
     }