[2.x] Move model traits.

[r-kujawa]

What does this PR do? 🤖

• Moves the ConfiguresCheckoutGateway trait to src/Traits.
• Extracts single use model traits content into their respective models.

Does this relate to any issue? 🔗

Closes #35

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 26.84%. Comparing base (c156ef8) to head (eafdc77).

Files	Patch %	Lines
src/Models/PaymentMethod.php	0.00%	6 Missing ⚠️
src/Models/PaymentTransaction.php	0.00%	6 Missing ⚠️
src/Traits/ConfiguresCheckoutGateway.php	0.00%	3 Missing ⚠️
src/Models/Wallet.php	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #48   +/-   ##
=========================================
  Coverage     26.84%   26.84%           
  Complexity       67       67           
=========================================
  Files            17       14    -3     
  Lines           149      149           
=========================================
  Hits             40       40           
  Misses          109      109           

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