Migrate Scan Operators to Scan Function

[andyfengHKU]

For the purpose of code maintenance, we should reduce the total number of operators in the code base. In particular, we should merge different scan operators into one operator and instead implement as different scan functions. If you think of all the scan operators/functions in the code base, there are

1. scan csv
2. scan parquet
3. scan rdf
4. scan numpy
5. scan pandas
6. scan factorized table
7. scan union all table
8. scan simple aggregate
9. scan hash aggregate
10. scan order by table
11. scan node id
12. scan column
13. scan csr

For 1-6 we have already moved them as scan function inside InQueryCall (we should rename this operator to Scan in the end). We should do the same for the rest scan functions. 7-10 are in memory tables so their implementation should be similar to 6. 11 - 13 involves scanning from storage so I would suggest whoever implement them discuss with @ray6080 first.