Add the symbols from Sample Node

Sampling table with `POISSONIZED` (such as `SELECT * FROM table TABLESAMPLE POISSONIZED (10) RESCALED;`) would get the error `java.lang.IllegalArgumentException: Key '$sampleweight' not present in map`. This issue was also reported before in #1307.

That problem is due to that the output symbols of Sample Node are not imported. This PR adds the symbols of Sample Node and fixes this problem.