Add MongoDB support

[AndreiKingsley]

[DestBro]

Since MongoDB is used sometimes for BigData things, a fast conversion of BSON Documents into a DataFrame should be a priority. Right now I do it like this:

@file:DependsOn("org.mongodb:mongodb-driver-kotlin-sync:5.5.1")

import com.mongodb.kotlin.client.MongoClient
import com.mongodb.kotlin.client.MongoDatabase

val connectionString = "mongodb://root:example@localhost:27017"
val client = MongoClient.create(connectionString = connectionString)

val databaseName = "db name"
val db: MongoDatabase = client.getDatabase(databaseName = databaseName)
var collection: MongoCollection<Document> = db.getCollection("comparison_result")

val dataframe = collection.find().allowDiskUse(true).map { DataFrame.readJsonStr(it.toJson()).flatten()  }.toList().toDataFrame().explode("value").flatten("value")

The current approach took about 4 minutes to load approximately 32,000 records (with nested arrays). I'm not sure excatly where is the bottleneck (network IO, json de/serialisation, etc) - because it was just an experiment.

Some comments about this solution from @AndreiKingsley :

As for your current approach — nice workaround! One small improvement you might consider: instead of fully deserializing and constructing manually, you can use DataRow.readJsonStr() on each row, and then combine everything using List.concat() to form the final DataFrame. That should simplify the code a bit and possibly improve performance too.

However I think that conversion of BSON to JSON format should not be part of the solution, maybe there is another way.