source-sqlserver: Consider populating /_meta/source/ts_ms based on cdc.lsn_time_mapping

[willdonnelly]

The /_meta/source/ts_ms property is supposed to hold the "Unix timestamp (in millis) at which this event was recorded by the database" if we have that information.

Currently for SQL Server this field is never populated because that information isn't part of the actual CDC change event table, however it would be possible in principle for us to fill it out by looking up change event LSNs against the cdc.lsn_time_mapping metadata.

We should investigate doing this, and probably profile the impact on typical connector throughput in order to decide whether it should be always-on or an optional feature. It feels like it ought to be possible to handle this fairly efficiently in the common case of small polling intervals, but I'm a bit concerned with how it might scale to larger catchup intervals that we'd see immediately after significant task downtime.