feat: crate for retry with backoff strategy

[vigith]

crate to do retries with different backoff strategies as an iterator for any async function.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.58140% with 21 lines in your changes missing coverage. Please review.

Project coverage is 51.20%. Comparing base (5f3766a) to head (a8d6e62).

Files	Patch %	Lines
serving/backoff/src/lib.rs	75.58%	21 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1785 +/- ## ========================================== - Coverage 55.94% 51.20% -4.75% ========================================== Files 274 275 +1 Lines 22174 26851 +4677 ========================================== + Hits 12406 13749 +1343 - Misses 8957 12293 +3336 + Partials 811 809 -2 ```

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[vigith]

This coverage report is wrong, it is checking doc comments and marking it as not covered even though I have doc tests.

   Doc-tests backoff

running 1 test
test backoff/src/lib.rs - (line 7) ... ok

[vigith]

we can do that in a follow up PR