Handle large SCN values in Offsets and SourceInfo block

For the OracleOffsetContext we should handle this seamlessly without needing to force the users to perform a re-snapshot. Simply add a new field that's meant to represent the SCN as a BigInteger and when loading the offsets, we check for the existence of the new field and if so, we use that value; if not we read the SCN from the legacy field. From here forward, only the new field will be written to.

For the SourceInfo block, there is no need to over-complicate this with a V1/V2 scheme like we've done in the past for connectors that weren't incubating. Here the suggestion would be to convert the field to a string and simply write the value as such.

The code contributed in DBZ-2855 used BigInteger to store the SCN in the logminer metadata queries; however it may be useful to use unsigned long from Java 8 instead, need to confirm with the team. If we go with the latter, it would make sense to synchronize this decision with the code changes included in the associated issue.