Re-introduce IndexModel from the official driver

[simoneromano96]

With mongo driver 2.0 they re-introduced the IndexModel.

Currently wither uses a custom implementation to create and manage indexes.

There is also a bug, the custom implementation does not work with text-based indexes after the first run, I don't think we should try to solve this issue before re-introducing the IndexModel since I'm pretty sure it will fix itself.

Related documentation:

• https://docs.rs/mongodb/2.0.0/mongodb/struct.IndexModel.html
• https://github.com/mongodb/mongo-rust-driver/releases/tag/v2.0.0 at Index Management API (RUST-273)