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/src/changes/get.rs

@@ -413,6 +413,7 @@ impl IntermediateChangesResponse {
             | MethodObject::SieveScript
             | MethodObject::Principal
             | MethodObject::Quota
+            | MethodObject::MaskedEmail
             | MethodObject::Registry(_) => unreachable!(),
         })
     }