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

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`.