Push notification extension

This change adds XDELTAPUSH capability.

Delta Chat clients detecting this capability
can set /private/devicetoken IMAP metadata
on the inbox to subscribe for Apple (APNS)
notifications.

Notifications are implemented in a new
`chatmail-metadata` service
which handles requests to set /private/devicetoken
IMAP metadata from Delta Chat clients
and /private/messagenew requests from
push_notification_lua script.

To avoid sending notifications for
MDNs, webxdc updates and Delta Chat sync messages,
messages with Auto-Submitted header are ignored
by setting $Auto keyword (flag) on them in Sieve script
and skipping such messages in push_notification_lua script.
Outgoing messages are also ignored.

chatmaild/pyproject.toml

@@ -10,6 +10,7 @@ dependencies = [
   "iniconfig",
   "deltachat-rpc-server",
   "deltachat-rpc-client",
+  "requests",
 ]
 
 [tool.setuptools]
@@ -20,6 +21,7 @@ where = ['src']
 
 [project.scripts]
 doveauth = "chatmaild.doveauth:main"
+chatmail-metadata = "chatmaild.metadata:main"
 filtermail = "chatmaild.filtermail:main"
 echobot = "chatmaild.echo:main"
 chatmail-metrics = "chatmaild.metrics:main"