Postgres Sink: support for `ON CONFLICT DO NOTHING`

[francoishornoy]

Is your feature request related to a problem? Please describe.
It would be useful to have a ON CONFLICT ... DO NOTHING option. Right now we only have 2 options: upsert or fail on collisions. It's hard to recover when we expect some duplicates once in a while, and rely on a unique constraint on database-side to reject duplicates.

Describe the solution you'd like
A new arg that allows to perform the INSERT with on conflict do nothing. The param could be a str in order to support the various usecases: f"ON CONFLICT {on_conflict_target} DO NOTHING".
Examples

• ON CONFLICT (pk_col1, pk_col2) DO NOTHING
• ON CONFLICT ON CONSTRAINT unique_constraint_name DO NOTHING
• ON CONFLICT DO NOTHING

@gwaramadze @tim-quix FYR.