Add emulation function for Snowflake's OBJECT_AGG()

[Hackerpilot]

No description provided.

[danieldeleo]

/gcbrun

[danieldeleo]

@Hackerpilot I think it'd be good to add unit tests that confirm it behaves exactly the same as snowflake:

Usage notes:

• Input tuples with NULL key and/or value are ignored.
• Duplicate keys within a group result in a Duplicate field key 'key' error.
• The DISTINCT keyword is supported, but it only filters out duplicate rows where both key and value are equal.

[Hackerpilot]

• "When a JavaScript UDAF is called with the IGNORE NULLS argument, BigQuery automatically skips over rows for which any aggregate argument evaluates to NULL. "
• This case is handled, but our testing functions currently don't have a way to specify that a function is expected to result in an error.
• This is now handled by a NOT AGGREGATE argument (which will be added automatically by the translation engine).

[danieldeleo]

Thanks for the changes!

[danieldeleo]

/gcbrun