feat: added a generic creditPurchase Url logic

[NoyanAziz]

This PR aims to introduce an override for creditPurchaseUrl based on an env variable CREDIT_PURCHASE_URL

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.60%. Comparing base (05a0810) to head (27e7020).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #675   +/-   ##
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  Coverage   97.60%   97.60%           
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  Files         152      152           
  Lines        1334     1337    +3     
  Branches      229      233    +4     
=======================================
+ Hits         1302     1305    +3     
  Misses         31       31           
  Partials        1        1           

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[deborahgu]

looks good. I'd like to see a test on the new logic, but if you want to hold off on the test until monday that's fine with me, because we see the fallback is still there.