cleanup(gax)!: group retry policy arguments

[coryan]

Use a struct for the retry policy arguments. This will make it possible
to add new fields to the struct without breaking any existing policies,
even those written by customers.

This could help with #2495, and it is a pre-requisite for making the retry policy APIs public.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.49%. Comparing base (9987f27) to head (642b95f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/storage/examples/src/lib.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3007      +/-   ##
==========================================
- Coverage   96.59%   96.49%   -0.10%     
==========================================
  Files         106      107       +1     
  Lines        4376     4396      +20     
==========================================
+ Hits         4227     4242      +15     
- Misses        149      154       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[dbolduc]

LGTM, I think this needs a rebase though to pick up the retry policy added to w1r3

[coryan]

PTAL.