Add axum-extra crate (#536)

* Add `axum-extra` crate

Empty for now but now we have a place to put stuff.

I'll make a PR for moving over https://github.com/davidpdrsn/axum-resource.

* remove `authors` field from `Cargo.toml`s

axum-extra/src/lib.rs

@@ -0,0 +1,43 @@
+//! Extra utilities for [`axum`].
+//!
+//! [`axum`]: https://crates.io/crates/axum
+
+#![warn(
+    clippy::all,
+    clippy::dbg_macro,
+    clippy::todo,
+    clippy::empty_enum,
+    clippy::enum_glob_use,
+    clippy::mem_forget,
+    clippy::unused_self,
+    clippy::filter_map_next,
+    clippy::needless_continue,
+    clippy::needless_borrow,
+    clippy::match_wildcard_for_single_variants,
+    clippy::if_let_mutex,
+    clippy::mismatched_target_os,
+    clippy::await_holding_lock,
+    clippy::match_on_vec_items,
+    clippy::imprecise_flops,
+    clippy::suboptimal_flops,
+    clippy::lossy_float_literal,
+    clippy::rest_pat_in_fully_bound_structs,
+    clippy::fn_params_excessive_bools,
+    clippy::exit,
+    clippy::inefficient_to_string,
+    clippy::linkedlist,
+    clippy::macro_use_imports,
+    clippy::option_option,
+    clippy::verbose_file_reads,
+    clippy::unnested_or_patterns,
+    rust_2018_idioms,
+    future_incompatible,
+    nonstandard_style,
+    missing_debug_implementations,
+    missing_docs
+)]
+#![deny(unreachable_pub, private_in_public)]
+#![allow(elided_lifetimes_in_paths, clippy::type_complexity)]
+#![forbid(unsafe_code)]
+#![cfg_attr(docsrs, feature(doc_cfg))]
+#![cfg_attr(test, allow(clippy::float_cmp))]