change the merge behavior when no primary key and merge key are specified to append

[rudolfix]

Feature description

When no primary key and no merge keys are defined, the staging dataset is appended to the destination dataset. A warning is issued.

Are you a dlt user?

Yes, I'm already a dlt user.

Use case

Current behavior, which is falling back to replace is not intuitive and may lead to a loss of data. The default dlt behavior is append so that would be expected. Also switching from append to merge and forgetting to set the keys will lead to data loss.

Also, which is more interesting case, this allows the users to do append via staging. So data is moved once all jobs for a given table chain is loaded (and for some databases - in transactional manner)

Proposed solution

1. skip delete part in sql merge job if no keys are defined
2. issue a warning in _verify_schema when no keys are defined

Related issues

No response