Fix: create the struct key properly in struct_extract_sql

[georgesittas]

The issue here was:

>>> sqlglot.parse_one("STRUCT_EXTRACT(foo, bar)").sql(dialect="spark")
'foo.``'

Notice how bar is dropped. The reason is that the identifier created in the following lines has an incorrect AST: there's a Column under the Indentifier node.

sqlglot/sqlglot/dialects/dialect.py

Lines 416 to 419 in cc0a6e2

	def struct_extract_sql(self: Generator, expression: exp.StructExtract) -> str:
	this = self.sql(expression, "this")
	struct_key = self.sql(exp.Identifier(this=expression.expression.copy(), quoted=True))
	return f"{this}.{struct_key}"