[FEATURE] collector automatically merge and align multiple `collect()` called with different schema

[badmonster0]

e.g.

collector.collect(field1=x, field2=y);
collector.collect(field1=x, field3=z);

The schema for the collector should have field1, field2 and field3.

During execution, we should also be able to align fields and fill null values for missing fields, and get a schema with fields field1, field2 and field3.