Add ability for backend engines to have "engine-specific behavior"

[richtia]

This came up in the following discussion: substrait-io/substrait#615 (comment)

For BFT / dialect testing I think we will need to add the ability for an engine to have an 
"engine-specific behavior"  which is defined as part of the dialect but not rooted in any 
substrait yaml. I think, for now, a minimal standard  of "at least 2 engines" will be a good guideline.