Replace proof_of_sql_parser::Identifier with sqlparser::ast::Ident in the proof-of-sql crate

[iajoiner]

Background and Motivation

This issue is a subtask in #235. In short since we plan to add more SQL features we plan to switch to the sqlparser crate which is a feature-rich, no_std-compatible parser used by DataFusion, which is part of the Arrow ecosystem.

Right now we already have some code that can convert intermediate AST from proof-of-sql-parser we use to sqlparser AST. Now we need to systematically replace instances of proof-of-sql-parser constructs with their corresponding sqlparser ones.

Changes Required

• Replace proof_of_sql_parser::Identifier with sqlparser::ast::Ident in the proof-of-sql crate.
• Do not add any constraint for identifier length. Any valid sqlparser::ast::Ident should be legit PoSQL identifiers from now on.