Make the HTTP client backend used by Bucket pluggable

Traits are defined which backends must implement: - The HTTP client `tower::Service` interface. - The `ResponseBody` that the backend must return. Minimal new dependencies are taken, notably `http-body` but not the whole `tower` crate, only the small crate with the trait definition. All 3 built-in backends, Reqwest, Surf, and Atto, implement the necessary traits. Exactly one of them is compiled depending on feature flags and exposed as `DefaultBackend` so that everything works the same as before by default. Future work will be 2 refactor some functionality such as retries and the `fail-on-err` feature as `tower::Layer` middleware. This is a somewhat breaking change and so will require a semver bump but I tried to keep it as unintrusive as possible. <!-- Reviewable:start --> - - - This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/durch/rust-s3/442) <!-- Reviewable:end --> <!-- This is an auto-generated comment: release notes by coderabbit.ai --> ## Summary by CodeRabbit ## Release Notes * **Breaking Changes** * Updated Bucket type and related public method signatures; code using Bucket will require updates. * Added new error type variant for invalid HTTP status codes. * **New Features** * Added new builder methods for fluent configuration of bucket settings. * Enhanced error handling with improved status code validation. _{✏️ Tip: You can customize this high-level summary in your review settings.} <!-- end of auto-generated comment: release notes by coderabbit.ai -->