add a property to tell timestamp extractor whether timestamp value is in seconds or milliseconds

[bagipriyank]

timestamp extractor assumes that timestamp value is in seconds which is causing an issue mentioned in https://github.com/confluentinc/kafka-connect-storage-common/issues/120#issuecomment-578459085

adding a property to tell timestamp extractor that timestamp is in seconds will help resolve this issue.

[nojetlag]

in general this should be much more flexible especially with financial data often timestamps are in microseconds.