chore(GODT-2799): Separate account states for SMTP Backend

Rather than accessing the Bridge user list, each user register their
individual SMTP service with the server manager.

Note that some dependencies on the user data are hidden behind the
`UserInterface`. These will be removed in a future patch.

internal/bridge/types.go

@@ -43,6 +43,11 @@ type Identifier interface {
 	GetClientString() string
 }
 
+type UserAgentUpdater interface {
+	Identifier
+	SetUserAgent(name, version string)
+}
+
 type ProxyController interface {
 	AllowProxy()
 	DisallowProxy()