Prevent frequent schema changes in MongoDB connector using Avro converter

When using MongoDB connector with Avro converter, and UnwrapFromMongoDbEnvelope, schema changes at a very frequent pace, the result is Too many schemas created error from schema-registry, or a slow down in Kafka-connect worker.

There are 2 reasons for this behavior:
1. Frequent schema changes in a mongoDB collection.
2. Changes in field order sent to schema-registry.

Proposed solution is to cache schema per topic, and add new fields to it as optional fields so schema will evolve to contain all fields, this will be done while keeping the order of the fields.