[MBL-1165] Add Query and Mutation for Checkout Flow

[mtgriego]

📲 What

Added the last couple query and mutations for the post campaign checkout flow, also a renaming of a variable

🤔 Why

They are needed for the flow for validations and actually doing the backing

🛠 How

Tested the mutation and query in staging, put them in the app and in the Apollo Client

📋 QA

Not implemented yet

Story 📖

MBL-1165

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (8207db8) 73.95% compared to head (a52f31a) 73.94%.

Files	Patch %	Lines
...com/kickstarter/models/CreatePaymentIntentInput.kt	0.00%	1 Missing :warning:
...om/kickstarter/models/PaymentValidationResponse.kt	0.00%	1 Missing :warning:

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1953      +/-   ##
============================================
- Coverage     73.95%   73.94%   -0.01%     
+ Complexity     1977     1976       -1     
============================================
  Files           343      344       +1     
  Lines         19818    19819       +1     
  Branches       2759     2759              
============================================
- Hits          14657    14656       -1     
- Misses         3545     3546       +1     
- Partials       1616     1617       +1     

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