feat(GODT-2799): SMTP Service

Refactor code to isolate the SMTP functionality in a dedicated SMTP
service for each user as discussed in the Bridge Service Architecture
RFC.

Some shared types have been moved from `user` to `usertypes` so that
they can be shared with Service and User Code.

Finally due to lack of recursive imports, the user data SMTP needs
access to is hidden behind an interface until the User Identity service
is implemented.

internal/user/errors.go

@@ -20,8 +20,6 @@ package user
 import "errors"
 
 var (
-	ErrNoSuchAddress     = errors.New("no such address")
-	ErrInvalidReturnPath = errors.New("invalid return path")
-	ErrInvalidRecipient  = errors.New("invalid recipient")
-	ErrMissingAddrKey    = errors.New("missing address key")
+	ErrNoSuchAddress  = errors.New("no such address")
+	ErrMissingAddrKey = errors.New("missing address key")
 )