NIFI-6934: Add support for Postgres flavour of UPSERT action

[sshcherbakov]

Description of PR

Enables Postgres flavour of UPSERT action; fixes bug NIFI-6934.

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[MikeThomsen]

We can't have vendor-specific logic like this in a common SQL processor. Consider writing an upsert generator class and throwing UnsupportedOperationException if the type is other than postgres.

[sshcherbakov]

UPSERT as such is supported not only by Postgres, but unfortunately there is no standard SQL syntax indeed (MySQL syntax is different).
Good idea about making implementation selectable (that's what generator means here I guess) and throwing UnsupportedOperationException.
It should also be possible to auto-detect the target SQL database flavour to select implementation automatically. That's some more work though.
Will try to wrap something up.

[pvillard31]

Implemented with #4350 and #4390.