Hi
I wrote this PR to make mongo-kafka support user defined topic separator.
currently, mong-kafka use "." to build topics, which might not be allowed in some companies. So it is better to make it configurable.

I introduced topic.separator param in MongoSourceConfig it is "." by default. if the user want use other separator to build topics, just config this param. (E.g “topic.separator”：“-”)

This pr i