fix: support MongoDB DNS seed list connection (#7136)

As described in [MongoDB Docs](https://docs.mongodb.com/manual/reference/connection-string/#dns-seed-list-connection-format), an additional connection string format called DNS Seed List Connection format can be used. As this is the default format for MongoDB Atlas, the hosted service of MongoDB, this should be available also for typeorm. 
The connection format is identified by the url-schema "mongodb+srv" and does not allow specifying a port.
Fixes #3347
Fixes #3133

src/driver/mongodb/MongoDriver.ts

@@ -438,11 +438,15 @@ export class MongoDriver implements Driver {
      * Builds connection url that is passed to underlying driver to perform connection to the mongodb database.
      */
     protected buildConnectionUrl(options: { [key: string]: any }): string {
-        const credentialsUrlPart = (options.username && options.password)
+         const schemaUrlPart = options.type.toLowerCase();
+         const credentialsUrlPart = (options.username && options.password)
             ? `${options.username}:${options.password}@`
             : "";
+        const portUrlPart = (schemaUrlPart === "mongodb+srv") 
+            ? "" 
+            : `:${options.port || "27017"}`;
 
-        return `mongodb://${credentialsUrlPart}${options.host || "127.0.0.1"}:${options.port || "27017"}/${options.database || ""}`;
+        return `${schemaUrlPart}://${credentialsUrlPart}${options.host || "127.0.0.1"}${portUrlPart}/${options.database || ""}`;
     }
 
     /**