Add clearing code to credit transfers

[samlown]

Summary

• add a generic clearing identifier to pay.CreditTransfer for national bank or branch clearing codes
• use cbc.Code for the IBAN, BIC, account number, and clearing identifier fields
• keep the descriptive bank name as a string
• regenerate the credit transfer JSON schema

This allows formats such as OIOUBL to represent a Danish bank registration number without overloading the bank name field. The same field can also represent concepts such as UK sort codes and US routing numbers.

Testing

• go test ./...
• go generate .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.95%. Comparing base (00506a5) to head (4897d4c).
⚠️ Report is 1 commits behind head on release-0.504.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-0.504.0     #920   +/-   ##
================================================
  Coverage            94.95%   94.95%           
================================================
  Files                  327      327           
  Lines                17668    17668           
================================================
  Hits                 16777    16777           
  Misses                 549      549           
  Partials               342      342           

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