vtgate: route using vindex for composite IN clause

[sougou]

Fixes #6609

The details of the fix are explained in the issue.

• Added a new Route opcode: SelectMultiEqual
• The Values field for the opcode contains the list of vindex input values to route on
• The plan builder uses a different code path for composite IN clause vs. the simple case.
• The plan works for nested composites also.
• If the operand values don't match or are non-trivial, we fall back to SelectScatter.