Refactor RequestRetrier into backoff and URI middleware

[advayakrishna]

Before this PR

RequestRetrier was responsible for both selecting which URI to try next as well as implementing backoff behavior. This change breaks up that functionality into two separate middlewares:

• URI middleware which selects a URI from a list of middlewares and adds it to the Request
• Backoff middleware which implements retry backoff when a URI has been attempted previously

Motivation behind making this change is to enable further work like:

• Cleaning up the URIScoringMiddleware API in the future, this can be used instead of the default URI middleware instead of in addition
• Making it easier to modify Request properties in middleware e.g. Add rendezvous hash URI scorer #268
• Provide consumers with clean APIs like Retrier or Middleware to add custom retry or URI selection logic to CGR

After this PR

==COMMIT_MSG==
Refactor RequestRetrier functionality into two separate middlewares. Backoff middleware implements retry backoff for URIs that have already been attempted and URI Middleware
==COMMIT_MSG==

Possible downsides?

No change in behavior or public API in this change so no apparent downsides.

---

This change is 

[changelog-app]

Generate changelog in changelog/@unreleased

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Refactor RequestRetrier functionality into two separate middlewares. Backoff middleware implements retry backoff for URIs that have already been attempted and URI Middleware

Check the box to generate changelog(s)

• Generate changelog entry

[stale]

This PR has been automatically marked as stale because it has not been touched in the last 14 days. If you'd like to keep it open, please leave a comment or add the 'long-lived' label, otherwise it'll be closed in 7 days.