source-sqlserver: Capture row preimage on updates

[willdonnelly]

Currently source-sqlserver doesn't capture row preimages at all when processing a CDC row update. This isn't as big a deal as you'd expect, but at some point somebody will probably want to capture before-state values as well.

This is done in SQL Server by specifying the all update old row filter option on the cdc.fn_cdc_get_all_changes_<instance> function instead of all like we currently specify, and adding some handling for __$operation values of 3 (which indicate an update preimage) to buffer them and combine with the subsequent postimage row.

We have similar logic for another connector, I think maybe Snowflake? It's not especially complicated, at least if we can assume that each preimage is immediately followed by the corresponding postimage.