Added MongoDB example

[tr1sm0s1n]

Hello! ^^

Motivation

Recently, I've been trying to create an API service in Rust to communicate with MongoDB using axum. Adding an example would be nice for future developers.

Solution

• Added a MongoDB example to examples/mongodb.
• Used the official MongoDB Rust Driver for the communication.
• Added routes to handle CRUD operations.

[jplatte]

Sorry for the long long delay on this. I haven't used MongoDB but David initally reviewed this, I guess there can't be much wrong with merging this.