Default to retry-after header value for 503 in ClientContextExtensions.cs

[tbithell]

Is your feature request related to a problem? Please describe.
Based on discussions I've had with various folks at Microsoft a 503 response should always honor the retry-after header value. The way 429s are handled is fine, but retrying a 503 sooner than the retry after header can result in being further penalized. Also, I've seen it vary as far as the value returned, sometimes being as low as 5 seconds, so in that case of a 503 it doesn't appear we are doomed to a 120 second back off.

Describe the solution you'd like
Handle the 503s separately from the 409s. In the case of the 503s always honor the retry-after header

[ghost]

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