physicalplan: preevaluate subqueries on LocalExprs

When the plan is local, we do not serialize expressions. Previously, in
such a case we would also not preevaluate the subqueries in the
expressions which made EXPLAIN (VEC) return unexpected plan (there
would tree.Subquery in the expression which we don't support in the
vectorized, so we would wrap the plan). Now we will preevalute the
subqueries before storing in the processor spec. AFAICT it affects only
this explain variant and nothing else.

Release note: None

colexec: improve expression parsing

This commit introduces colexec.ExprHelper that helps with expression
processing. Previously, we were allocating a new execinfra.ExprHelper
and were calling Init on it in order to get the typed expression from
possibly serialized representation of each expression. Now, this new
expression helper is reused between all expressions in the flow on
a single node.

There is one caveat, however: we need to make sure that we force
deserialization of the expressions during SupportsVectorized check if
the flow is scheduled to be run on a remote node (different from the one
that is performing the check). This is necessary to make sure that the
remote nodes will be able to deserialize the expressions without
encountering errors (if we don't force the serialization during the
check, we will use LocalExpr - if available - and might not catch
things that we don't support).

Release note: None

physicalplan: always store LocalExpr

Previously, we would set either LocalExpr (unserialized expression,
only when we have the full plan on a single node) or Expr (serialized
expression, when we have distributed plan as well as in some tests).
However, we could be setting both and making best effort to reuse
unserialized LocalExpr on the gateway even if the plan is distributed.
And this commit adds such behavior.

Fixes: #49810.

Release note: None