Clarify docs that PayPalNativeShippingListener is sometimes required

[scannillo]

Equivalent iOS PR - https://github.com/paypal/iOS-SDK/pull/150

Reason for changes

• This GH issue raised a question about when a merchant needs to implement the shipping callback. Turns out, it is not entirely optional. There is a case when a merchant must set it to avoid an infinite spinner on their checkout button.

Summary of changes

• Clarify that merchants using GET_FROM_FILE orders must implement PayPalNativeShippingDelegate

Checklist

• Added a changelog entry

Authors

@scannillo

[codecov]

Codecov Report

Merging #140 (ff5da9a) into main (25bc012) will not change coverage.
The diff coverage is n/a.

❗ Current head ff5da9a differs from pull request most recent head 6877aed. Consider uploading reports for the commit 6877aed to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #140   +/-   ##
=========================================
  Coverage     32.55%   32.55%           
  Complexity       20       20           
=========================================
  Files            22       22           
  Lines           301      301           
  Branches         21       21           
=========================================
  Hits             98       98           
  Misses          197      197           
  Partials          6        6           

Flag	Coverage Δ
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[scannillo]

cc: @jcnoriega Can you take a look?

[wallportillo]

@scannillo these look good to me