Chatmail: IMAP METADATA (RFC 5464) extension support

[s0ph0s-dog]

This is part of a series of issues tracking chatmail support in Mox.

Chatmail servers support push notifications for iOS and Android through a work around that requires the cooperation of the mail server. Delta Chat clients set push notification tokens by using the IMAP SETMETADATA command to inform the server what the notification token is. The server then sends a POST request to notifications.delta.chat which includes the notification token in the body. The notifications service then uses the Apple or Google push notification APIs to prod client devices to wake up and check for new emails. Delta Chat also looks at another metadata key to see if the server provides an Iroh relay for real-time P2P message passing in WebXDC applications.

In order to enable this functionality, Mox needs support for the RFC 5464 IMAP METADATA extension. This will require support for all of the behaviors described in the specification, but it also requires enough programmatic extension points for other parts of Mox to react when metadata keys are set or read.

[mjl-]

I'll give implementing IMAP METADATA a try this week!

[mjl-]

The commit just now is the first part. It only allows setting/getting annotations in the /private/* namespace. I think you'll need a way for an admin to set /shared/* annotations too, and let users read it. Not sure where to put those values: config file, or a database file with access through cli and/or admin page. Multiline values in the config file are annoying, I suspect a database file would be better.