Allow robust to use the server-suggested Retry-After header

[malmans2]

Description

The Retry-After HTTP response header indicates how long the user agent should wait before making a follow-up request. This PR allows to use the server-suggested retry_after value and is implemented as an opt-in feature to avoid breaking changes.

The HTTP specification allows Retry-After to be provided either as a delay in seconds or as a date. I believe the latter is out of scope for this use case, which motivates the try/except used to convert the string to a float.

Side note: The Data Stores Service does provide a Retry-After value with 429 errors.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 39.13043% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 14.93%. Comparing base (acdbb59) to head (635c8c1).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
multiurl/http.py	0.00%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #37      +/-   ##
===========================================
+ Coverage    14.35%   14.93%   +0.58%     
===========================================
  Files           14       14              
  Lines         1059     1078      +19     
===========================================
+ Hits           152      161       +9     
- Misses         907      917      +10     

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