Use fallback API hosts when receiving server down response

[ajpallares]

Checklist

• If applicable, unit tests
• If applicable, create follow-up issues for purchases-ios and hybrids

Description

This PR implements a retry mechanism using fallback URL hosts for supported requests that fail with a server down error

Notes

• Only some requests support a fallback API host. Currently only getCustomerInfo and getProductEntitlementMapping support it.
• AppConfig now has a fallbackBaseURLs containing the list of hosts, in order of preference, to use for supported requests.
• When a request's response is a server error (5XX), the same request is immediately retried with the next host of the array.
• The logic is implemented per-request: that is, if a request fails with a server-down error for host 1 and then falls back and succeeds for host 2, then all future requests will use again host 1 initially. No state is held between different requests.
• No fallback retries happen when using a proxy URL

iOS PR counterpart --> RevenueCat/purchases-ios#4970

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 9 lines in your changes missing coverage. Please review.

Project coverage is 79.93%. Comparing base (be4ef49) to head (f47b681).

Files with missing lines	Patch %	Lines
...revenuecat/purchases/common/networking/Endpoint.kt	42.85%	0 Missing and 8 partials ⚠️
...tlin/com/revenuecat/purchases/common/HTTPClient.kt	96.29%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2368      +/-   ##
==========================================
+ Coverage   79.92%   79.93%   +0.01%     
==========================================
  Files         284      284              
  Lines       10151    10204      +53     
  Branches     1433     1448      +15     
==========================================
+ Hits         8113     8157      +44     
  Misses       1422     1422              
- Partials      616      625       +9     

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