add retries to fastly api requests, use same retry logic for registry api

[syphar]

to fix errors like this sentry error

• fastly api requests weren't retried before, we have some sentry errors where content was outdated because of that.
• registry api retries where just retried for all possible errors. the reqwest-retry logic only retries transient errors, which would reduce load on crates.io.

With this middleware we get retries just for "transient" errors, and niceties like jittering.

When the repackage & #3414 is done we then can remove our custom retry_async method.

the default retry strategy is explained here:

• https://docs.rs/reqwest-retry/latest/reqwest_retry/fn.default_on_request_success.html
• https://docs.rs/reqwest-retry/latest/reqwest_retry/fn.default_on_request_failure.html

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.