impl(auth): make TokenProviderWithRetry RefUnwindSafe

[alvarowolfx]

The TokenProviderWithRetry struct was not implementing RefUnwindSafe because it held Arc<dyn RetryPolicy> and Arc<dyn BackoffPolicy>, which are trait objects that do not automatically implement RefUnwindSafe.

When TokenProviderWithRetry was embedded into other public structs (like IDTokenCredentials), those structs also lost their RefUnwindSafe implementation. This caused a semantic versioning check failure as RefUnwindSafe is part of the public API promise.

Towards #3592

[alvarowolfx]

unblocks #4260, #4257 and #4256

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.04%. Comparing base (90873cc) to head (fe2f63c).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/auth/src/retry.rs	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4447      +/-   ##
==========================================
- Coverage   95.04%   95.04%   -0.01%     
==========================================
  Files         194      194              
  Lines        7372     7384      +12     
==========================================
+ Hits         7007     7018      +11     
- Misses        365      366       +1     

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[coryan]

If we are going to do this, I propose we simply implement the trait for TokenProviderWithRetry (or the builder for it).

[alvarowolfx]

It's an option too. I did this because only retry_policy and backoff_policy are not UnwindSafe, but you're right, in the end we are adding a impl RefUnwindSafe thing anyway.

[alvarowolfx]

hmm, weird. I had to add both RefUnwindSafe and UnwindSafe explicitly to make it work:

impl RefUnwindSafe for Builder {}
impl UnwindSafe for Builder {}