MM-43918: freemium limit for storage (#20225)

Summary
Adds an API end point to return storage usage

Ticket Link
https://mattermost.atlassian.net/browse/MM-43918

model/usage.go

@@ -7,6 +7,10 @@ type PostsUsage struct {
 	Count int64 `json:"count"`
 }
 
+type StorageUsage struct {
+	Bytes int64 `json:"bytes"`
+}
+
 type TeamsUsage struct {
 	Active        int64 `json:"active"`
 	CloudArchived int64 `json:"cloud_archived"`