[feature] fallback to kafka topic name

[hpgrahsl]

The current collection/topic aware configuration options expect to provide an explicit mapping between any kafka topic name and its corresponding mongodb collection name. Besides, there is the option to specify a default mongodb collection name in case no explicit mapping is provided but this means that all sink records of potentially different kafka topics are written to the same mongodb collection. Complex connector pipelines would almost always specify proper mappings for all kafka topics in question especially since most likely different sink connector behaviour is needed for processing the contained sink records.

However, for very simple pass-through kinds of streaming ETL pipelines from kafka to mongodb it might be useful to allow to fallback not to a single default topic name but to use the name of the kafka topic as is for the mongodb collection name.

[hpgrahsl]

implemented in #68