Implement HAVING MIN/MAX and RESPECT/IGNORE NULLS

[apstndb]

This PR implements the current aggregate function call syntax.

https://cloud.google.com/spanner/docs/reference/standard-sql/aggregate-function-calls#aggregate_function_call_syntax

function_name(
  [ DISTINCT ]
  function_arguments
  [ { IGNORE | RESPECT } NULLS ]
  [ HAVING { MAX | MIN } having_expression ]
)

Note

• The names NullHandlingModifier and HavingModifier are borrowed from ZetaSQL
• COUNT(*) doesn't support these modifiers, so CountStarExpr is not updated.

fix #88

[apstndb]

Conflict resolved and suggestion applied.

[makenowjust]

@apstndb Thank you!