CosmosDB - Attachments support

This release adds attachments support (both reference and slug) to the Cosmos DB crate. There are few small breaking changes (most notably the ConsistenctLevel enum now supports a Cow instead of a raw &str for better ergonomics so it loses the Copy auto trait). You should not be impacted by the changes, however (I hope!).
All the attachments methods are supported and there are E2E tests as well. As usual, please refer to the examples folder for how to use it (as this time there is this example: attachment00.rs).