Masked email: Fastmail's Masked Email API, MaskedEmail/get and /set (ME-1, ME-7a, ME-16)

Advertised as https://www.fastmail.com/dev/maskedemail in the session and on
every account that may hold masks. Masks created through it start pending
unless the create sets a state; pending can't be set again once left; state
and the other mutable fields map onto the same records the x: API uses.

crates/jmap-proto/src/object/mod.rs

@@ -18,6 +18,7 @@ pub mod calendar_event_notification;
 pub mod contact;
 pub mod email;
 pub mod email_submission;
+pub mod fastmail_masked_email; // inbuxa: masked email
 pub mod file_node;
 pub mod identity;
 pub mod mailbox;