* refactor: split SQLPage functions into one module each
Each built-in `sqlpage.*` function is now a plain `async fn` in its own
file under `sqlpage_functions/functions/`, with an ordinary Rust signature
and no marker comments or macros inside it.
Registration is automatic: `build.rs` lists the files in `functions/` into a
`sqlpage_functions!` call (the only generated code, one line per function),
and that macro declares the modules and builds the `SqlPageFunctionName`
enum the SQL engine dispatches on. There is no marker-comment parsing and no
per-argument codegen. Argument extraction, dispatch and return-value
conversion are ordinary generic code in function_traits.rs (`Extract`,
`Handler`, `IntoCowResult`), using the same `Fn`-arity trick axum uses for
handlers, so a function's argument and return types are read straight from
its signature.
Also folds in main's FileAccess centralization (#1327) for the file-reading
functions.
Verified: `cargo test` passes (148 lib + 70 integration tests, including
run_all_sql_test_files, the hmac webhook tests, the upload tests and
test_file_upload_through_runsql which exercises the &mut DbConn path).
`cargo clippy` and `cargo fmt --check` are clean.
* less magic
Prayag Bhakar