Support Join pushdown with coercions

[findepi]

Support Join pushdown when joining eg bigint column with integer column, or when joining varchar(n) with varchar(m) column.

[wendigo]

Is this something to be decided by the connector (we could push types for both sides of the join and connector will decide)? Or should we make it at the engine level?

[findepi]

Currently join pushdown is not attempted at all.
Coercion prevents PushJoinIntoTableScan from even firing up.
We would also need a way to represent CAST in ConnectorExpression.
CAST can be represented as a function call (with some special name) or as an explicit CAST.
The former gives uniformism (cast is a function after all), but the latter would be easier to handle in practice,
and also would not tie into any runtime function transactional resolution problems or anything like that.

cc @losipiuk @martint

[RameshByndoor]

This is much useful, Most of the queries falls into this category and we see join not getting pushdown.

[sumannewton]

This might be helpful in fetching the wider data type among similar data types.