Initial work on adding operator map

[tanclary]

No description provided.

[julianhyde]

There should be an auxiliary call 'RexNode transform(RexCall call, SqlDialect targetDialect)'. For a given operator, it should look for an equivalent operator that is avaialble in the target dialect, and convert the call into a call to the new operator.

Some operators - e.g. DATEDIFF - would permute the operands.

The BiMap is a nice idea, but I think it might run out of steam. I would have one map, and a 'registerBijection' method that would put two entries into the map, one for each target dialect.

[github-actions]

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