Add toasty-mongodb

[bsodmike]

This could be quick given there is toasty-dynamodb already, but the underlying crates would need to be changed.

[nikhil-pandey]

+1

[JJPuck]

We have a library very similar to toasty at work that we've considered open sourcing. It primarily targets mongodb. If there's interest I could look at porting it across, though it uses derive macros rather than code generation ahead of time.