Extend index definition in Defined Schema

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

In Defined Schema, only simple compound indexes can be defined, the following index options cannot be defined:

• partialFilterExpression
• sparse
• ttl
• unique

Feature / Enhancement Description

Add support for the above mentioned index options. These options do not have to be added explicitly, instead a general field ofr index options should be added to the index definition.

Example Use Case

To avoid a breaking change, index options could be added by using an array instead of an object:

indexes: {
  index_without_option: {
    location: "2dsphere"
  },
  index_with_option: [{
    location: "2dsphere"
  }, {
    sparse: true
  }]
}

Alternatively for type consistency with breaking change:

indexes: {
  index_without_option: {
    fields: {
      location: "2dsphere"
    }
  },
  index_with_option: {
    fields: {
      location: "2dsphere"
    },
    options: {
      sparse: true
    }
  }
}

Start by locating the Defined Schema index-definition entry point and its existing tests. Review how compound indexes are represented and compare the proposed backward-compatible array form with the alternative fields/options form. Done means the supported MongoDB index options can be expressed without breaking existing definitions and are covered by tests.