Allow workload subcommand to write to real db

[bobvawter]

The workload command only targets the webhook endpoints at the moment. It would be useful if it could also write to a source db and run the checks against a separate target database.

[ryanluu12345]

So if I'm understanding this context correctly:

1. Write data to the source database via INSERTS/UPSERTS
2. Check the data on the source and target sides?

One thing I can think of here is to support 1 and then use molt verify for part 2 since it's built for comparing source and target data

[bobvawter]

I'm finalizing a PR that uses the workload generator and the apply package running against the source cluster for pglogical. Once that's in, the workload subcommand should be mostly copy-and-paste. The workload.Checker already has post-copy validation which could be leveraged.