add CreditNote and CustomerBalanceTransaction

[eric-seppanen]

Summary

Adds support for CreditNote and CustomerBalanceTransaction objects, which are different ways of applying credit to a customer account.

I wasn't sure whether CustomerBalanceTransaction actions (create, list, etc.) should be methods of Customer or of CustomerBalanceTransaction; since they have the customer id in the API path, I chose to make them part of Customer but left them in a separate module customer_balance_transaction_ext.rs. It would be easy to change either of those decisions if you disagree.

Note: this adopts 3 previously-unused generated modules, which means that cargo make fmt saw them for the first time. I'm not sure if committing those formatting changes is the right step, but I included that as a separate commit.

Checklist

• [x] ran cargo make fmt
• [x] using conventional commits to hightlight user-facing fixes and features

[arlyon]

This looks great as well. No comments really, I think the ext file is in an ok place and the formatting is expected.

[codecov[bot]]

Codecov Report

Merging #350 (3fe9627) into master (dbcff41) will decrease coverage by 0.16%. The diff coverage is 1.72%.

@@            Coverage Diff            @@
##           master    #350      +/-   ##
=========================================
- Coverage    6.50%   6.35%   -0.16%     
=========================================
  Files         131     136       +5     
  Lines       20243   20774     +531     
=========================================
+ Hits         1317    1320       +3     
- Misses      18926   19454     +528     

Impacted Files	Coverage Δ
src/client/base/async_std.rs	89.42% <0.00%> (ø)
src/error.rs	14.28% <ø> (ø)
src/ids.rs	56.33% <ø> (ø)
src/lib.rs	100.00% <ø> (ø)
src/resources/checkout_session_ext.rs	0.00% <0.00%> (ø)
src/resources/credit_note_ext.rs	0.00% <0.00%> (ø)
src/resources/customer_balance_transaction_ext.rs	0.00% <0.00%> (ø)
src/resources/generated/account.rs	4.04% <0.00%> (ø)
src/resources/generated/balance.rs	0.00% <ø> (ø)
src/resources/generated/billing_portal_session.rs	0.00% <0.00%> (ø)
... and 85 more

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