postgresql: support skipping conflicts when using COPY

[fitnr]

Feature description

Postgresql 17 added support for ignoring conflicts when loading data via copy using the ON_ERROR option:

COPY table FROM stdin WITH (ON_ERROR ignore);

Since COPY is a faster way to load data, supporting this would speed up many database operations.

Right now the OGR_PG_SKIP_CONFLICTS option only functions when PG_USE_COPY=NO, perhaps this feature could be supported when both are set to YES.

On the face of it, support would entail small updates to OGRPGTableLayer::StartCopy and OGRPGDumpLayer::StartCopy.

Additional context

No response