Allow to create unique indexes

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Current Limitation

parse-server is currently lacking a way to update DB schema. #7091 should address most of this issue, but unfortunatly, it does not seems like there is a way to add uniqueIndex despite being able to do so with the mongo client and postgre

db.collection.createIndex( <key and index type specification>, { unique: true } )

Feature / Enhancement Description

Add a isUniqueIndex parameter

SchemaController.createIndex(className, index , isUniqueIndex) {} 

Or even better, add an options parameter. The downside of this would be that itwould probably cause problems in inter-compatibility between mongo and postgre

 createIndex(className, index , options)

Example Use Case

schema.createIndex("_User" , {username:1} , true)
// or
schema.createIndex("_User" , {username:1} , {unique: true} )

Alternatives / Workarounds

We need to bypass the Parse Schema logic and implement it with manual migrations ... its a real pain !

3rd Party References

Start at the SchemaController.createIndex entry point named in the issue and trace how schema indexes are handled for MongoDB and PostgreSQL. Compare the proposed boolean and options forms, then define completion as a consistent way for schema.createIndex to request unique indexes across the supported databases, with coverage for the resulting behavior.