feat: experimental header protection for Autocrypt

This change adds support for receiving
Autocrypt header in the protected part of encrypted message.

Autocrypt header is now also allowed in mailing lists.
Previously Autocrypt header was rejected when
List-Post header was present,
but the check for the address being equal to the From: address
is sufficient.

New experimental `protect_autocrypt` config is disabled
by default because Delta Chat with reception
support should be released first on all platforms.

src/context.rs

@@ -990,6 +990,12 @@ impl Context {
                 .await?
                 .to_string(),
         );
+        res.insert(
+            "protect_autocrypt",
+            self.get_config_int(Config::ProtectAutocrypt)
+                .await?
+                .to_string(),
+        );
         res.insert(
             "debug_logging",
             self.get_config_int(Config::DebugLogging).await?.to_string(),