Merkle tree storage in MongoDB for zkDatabase as a service

[chiro-hiro]

Hi @magestrio,

I mentioned in the meeting that we going implement zkDatabase as a service that's why you see this package appear in our repo https://github.com/orochi-network/zkDatabase/tree/main/packages/serverless. To make this a production ready stage we need to store the merkle tree to MongoDB. It would be great to implement StorageEngineMongoDB.