opt: add rule to decorrelate union operators

[DrewKimball]

opt: improve EnsureKey custom function

This commit makes a small improvement to the EnsureKey custom function
(used in decorrelation rules), so that it can add passthrough columns to
a Project operator in an effort to find a key. This can prevent rules
from adding unnecessary Ordinality operators to the query plan.

Epic: None

Release note: None

opt: improve exists subquery hoisting

This commit makes a small improvement to the subquery-hoisting rules so
that hoisting an EXISTS subquery can often avoid projecting a new
column to check for NULL values. This can allow other optimization rules
to match later on.

Epic: None

Release note: None

opt: add rule to decorrelate unions in EXISTS subqueries

This commit adds a new rule TryDecorrelateUnion, which matches on a
Union or UnionAll operator in the input of a ScalarGroupBy. The
ScalarGroupBy must have "any-not-null" semantics, meaning it produces
an arbitrary non-null value from each input column.

If these conditions are satisfied, the Union operator is replaced by an
InnerJoin between two ScalarGroupBy operators. A Project coalesces
columns from each side of the join to produce the final aggregated values.

This transformation does not itself decorrelate the Union operators, but
it does make it easier for other rules to do so.

Release note: None

Epic: None

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[cockroach-teamcity]

This change is