[Refactor] Improve query structure nodes

[colinthebomb1]

Overview:

This PR improves the node ast

Code Changes:

• Added alias to FunctionNode, allows for cases like SELECT COUNT(*) as emp_count
• Changed set to list in Select, From, Where, and Having nodes, allowing for order of items in clauses to be maintained
• Added sort parameter in OrderBy node and created OrderByItemNode to allow more functionality in ORDER BY statements

[HazelYuAhiru]

Code Changes for the Parser

• Added __eq__ and __hash__ methods to several classes to enable proper comparison in parser test cases.
• Added a simple Join node structure for the Join required in the parser baseline test case.

@colinthebomb1 Could you take a look at the new code I added? Thanks a lot!

[baiqiushi]

Same as above. Since sort approach (DESC, ASC) are two SQL keywords, should we also use enum?

[colinthebomb1]

done

[baiqiushi]

What is the difference between OrderByItemNode and OrderByNode?

[colinthebomb1]

OrdrByItemNode is for single items in ORDER BY clauses like
OrderByItemNode(ColumnNode("salary"), _sort = SortOrder.DESC).
OrderByNode is used to build full ORDER BY clauses formed by multiple OrderByItemNode like

OrderByNode([
    OrderByItemNode(ColumnNode("salary"), _sort=SortOrder.DESC), 
    OrderByItemNode(ColumnNode("age"), _sort=SortOrder.ASC)
])

[colinthebomb1]

@HazelYuAhiru, other than the JoinType enum that Qiushi pointed out, your changes LGTM! I added the eq and hash for the nodes I was working on, too.