Combined Database backend: Add Static Account support to MongoDB (#7003)

* Implement SetCredentials for MongoDB, adding support for static accounts

* rework SetCredentials to split from CreateUser, and to parse the url for database

* Add integration test for mongodb static account rotation

* check the length of the password results to avoid out-of-bounds

* remove unused method

* use the pre-existing test helper for this. Add parse method to helper

* remove unused command

plugins/database/mongodb/util.go

@@ -5,6 +5,7 @@ type createUserCommand struct {
 	Password string        `bson:"pwd"`
 	Roles    []interface{} `bson:"roles"`
 }
+
 type mongodbRole struct {
 	Role string `json:"role" bson:"role"`
 	DB   string `json:"db"   bson:"db"`