[PATCH] Allow specification of custom slot for custom nodes

Hackers,

we have supported custom nodes for while. Custom slots are a bit more
"recent" feature. Since we now support custom slots, which could
handle custom tuple format, why not allow custom nodes to use them?

For instance, a custom table access method can have its own tuple
format and use a custom node to provide some custom type of scan. The
ability to set a custom slot would save us from tuple format
conversion (thank happened to me while working on OrioleDB). I think
other users of custom nodes may also benefit.

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Regards,
Alexander Korotkov