CosmosDB: Support for User Defined Functions

This release adds support for user defined functions (UDFs) to the CosmosDB crate.

Breaking

This is a breaking version as it changes how the Document is created and defined. Previously the Document had the notion of id field, treated separately from the rest of the serialized struct. This made for good APIs but, in practice, clashed with the fact that not every query returns a Document (as seen in the UDFs example). Now the Crate does not force any id so it's up to you to include it if you want. Be mindful that Azure CosmosDB will add an id field even if you do not (as of now a GUID but it can change) so I suggest you to handle it.