add option in collection config for advanced indexes management

[karthage-spartacus]

Hi,

it would be great to support advanced index definition at a collection level.
I'm using MongoDB as database and I often used to have compound indexes (one index on multiple keys) and localized indexes (on text definitions).
I'm also heavily relying on json fields in my projects, and i'd like to be able to index nested keys inside these json fields.
Right now i'm manually creating these indexes by adding them on the targeted DB, but native support would be awesome.

I know there is a discussion for compound indexes here : #1121 but it's on a field definition level.

Something like :

export const Users: CollectionConfig = {
  slug: 'users',
  access: {
    ...
  },
  fields: [
    ...
  ],
  advancedIndexes: [{
       name: 'my-index',
       keys: {
          foo: 1,
          bar: -1
       },
       options: { unique: true, ... }
  }]
}

Thanks a lot !