Add summary support for functions invoked via `CALL`

[duckinator]

(Opening this to track some of my future work.)

This is the test from pg_query.rs that depends on this functionality:

#[test]
fn it_finds_functions_invoked_with_CALL() {
    let result = summary("CALL testfunc(1);", 0, -1).unwrap();
    assert_eq!(result.warnings.len(), 0);
    assert_eq!(result.tables().len(), 0);
    assert_eq!(result.functions(), ["testfunc"]);
    assert_eq!(result.ddl_functions().len(), 0);
    assert_eq!(result.call_functions(), ["testfunc"]);
    assert_eq!(result.statement_types(), ["CallStmt"]);
}