Document usage of tower middleware like retry

[onbjerg]

Component

providers, pubsub, transports

Describe the feature you would like

We should explain that the transports are tower services and how to layer things like retries/backoff (with a simple example) in either the provider or transport docs (or both).

Additional context

No response

[zerosnacks]

I think because there are already quite a few references

alloy/crates/provider/src/builder.rs

Lines 15 to 17 in f6ebef2

	/// A layering abstraction in the vein of [`tower::Layer`]
	///
	/// [`tower::Layer`]: https://docs.rs/tower/latest/tower/trait.Layer.html

to Tower layers and

alloy/crates/transport/src/trait.rs

Lines 21 to 23 in f6ebef2

	/// This trait is blanket implemented for all appropriate types. To implement
	/// this trait, you must implement the [`tower::Service`] trait with the
	/// appropriate associated types. It cannot be implemented directly.

and Tower services it will be sufficiently covered once the retry-layer example is merged and available as a practical example alloy-rs/examples#95 cc @yash-atreya

[onbjerg]

i was more thinking around using built-in tower layers, or an example showing how to build and use a tower layer. but ultimately defer to you guys if you think #849 is sufficient:)

[gakonst]

i agree i'd like to have a "real" tower layer used here too