fix(purchase): Set type of `paymentDetails` to generic JSON

[ghost]

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#### [](https://app.codesee.io/r/reviews?pr=489&src=https%3A%2F%2Fgithub.com%2FAnalogIO%2Fcoffeecard_app) #### Legend

[codecov[bot]]

Codecov Report

Merging #489 (c0fde8b) into develop (b79b60e) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #489   +/-   ##
========================================
  Coverage    72.19%   72.19%           
========================================
  Files          108      108           
  Lines         1158     1158           
========================================
  Hits           836      836           
  Misses         322      322           

Flag	Coverage Δ
unittests	72.19% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...es/purchase/domain/entities/initiate_purchase.dart	100.00% <ø> (ø)
.../purchase/data/models/initiate_purchase_model.dart	100.00% <100.00%> (ø)
.../purchase/data/repositories/mobilepay_service.dart	68.75% <100.00%> (+1.00%)	:arrow_up: