Parameter for cdc with only snapshot

[Amogh-Bharadwaj]

This PR introduces a parameter - initial_copy_only which when set, will initiate CDC and exit after Snapshot Flow is complete. No Sync Flows will be called. By default this is set to false.

In the UI, this parameter is listed in the Advanced Settings section:

Validation for this parameter is set in both query layer and UI:

amogh=> CREATE MIRROR sfnosnappy1                                                                                                                                                                                                              
amogh-> FROM postgres_local TO sf_peer
amogh-> WITH TABLE MAPPING (
amogh(> public.oss2:public.oss2nexusso6,
amogh(> public.oss2ud1:public.oss2ud1nexusso6
amogh(> )
amogh-> WITH(
amogh(> do_initial_copy = false,
amogh(> snapshot_staging_path = '',
amogh(> initial_copy_only = true
amogh(> );
ERROR:  initial_copy_only is set to true, but do_initial_copy is set to false

This has been tested via UI and query layer and the happy path where it is not set it also tested.