feat: make RetriableServerErrorStatusCodes public

[maxkoshevoi]

Fixes

Made RetriableServerErrorStatusCodes public. This is useful if user wants to implement custom retry logic (in my case it's rescheduling bus message), or amend existing list (don't have an example for this one, but it also might be useful)

Checklist

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• I have read the Contribution Guidelines and my PR follows them
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• I have added tests that prove my fix is effective or that my feature works
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• I have added inline documentation to the code I modified

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[eshanholtz]

Hi @maxkoshevoi thanks for the PR! Can you please address the test failure and add some additional testing around the new public capabilities here?

[maxkoshevoi]

Turns out .Net Framework 4.0 doesn't have IReadOnlyList. Replaced it with regular List

[childish-sambino]

LGTM