Add ASYNC keyword support for CREATE INDEX

[amaksimo]

Summary

Amazon Aurora DSQL uses CREATE INDEX ASYNC to create indexes asynchronously. This adds parsing support for the ASYNC keyword in CREATE INDEX statements, following the same pattern as the existing CONCURRENTLY keyword.

Syntax: CREATE [UNIQUE] INDEX [ASYNC] [IF NOT EXISTS] <name> ON <table>(<columns>)

Changes

• Add ASYNC to the keyword list (keywords.rs)
• Add pub r#async: bool field to CreateIndex struct (ast/ddl.rs)
• Parse ASYNC keyword after CONCURRENTLY in parse_create_index (parser/mod.rs)
• Round-trip support via Display impl
• Update Spanned impl (ast/spans.rs)
• Tests for CREATE INDEX ASYNC and CREATE UNIQUE INDEX ASYNC (sqlparser_postgres.rs)
• Update all existing CreateIndex test patterns with the new field

Thanks for revewing!

[iffyio]

Suggested change

	let sql = "CREATE INDEX ASYNC my_index ON my_table(col1)";
	match pg().verified_stmt(sql) {
	Statement::CreateIndex(CreateIndex {
	name: Some(ObjectName(name)),
	table_name: ObjectName(table_name),
	using,
	columns,
	unique,
	concurrently,
	r#async,
	if_not_exists,
	include,
	nulls_distinct: None,
	with,
	predicate: None,
	index_options,
	alter_options,
	}) => {
	assert_eq_vec(&["my_index"], &name);
	assert_eq_vec(&["my_table"], &table_name);
	assert_eq!(None, using);
	assert!(!unique);
	assert!(!concurrently);
	assert!(r#async);
	assert!(!if_not_exists);
	assert_eq_vec(&["col1"], &columns);
	assert!(include.is_empty());
	assert!(with.is_empty());
	assert!(index_options.is_empty());
	assert!(alter_options.is_empty());
	}
	_ => unreachable!(),
	}
	pg().verified_stmt("CREATE INDEX ASYNC my_index ON my_table(col1)");

same for the unique one. Since the flag is already asserted in other tests, a simple round trip test scenario should suffice.

Also can we move this test to common.rs - since its not guarded specifically to postgres

[iffyio]

Can we add a link to the postgres docs describing the syntax?