Pay Upon Invoice implementation

[GytisZum]

Self-Checks

• I have performed a self-review of my code.
• I have updated/added necessary technical documentation in the README file.

JIRA task link

Resolves: XXXX

Summary

• New Payment Source Builder: Added PuiPaymentSourceNodeBuilder to
  construct PUI-specific payment payloads with customer billing details,
  phone number validation, and BIC requirements
• Order Creation Flow: Enhanced order creation logic to handle async
  capture requirements (PUI orders require ORDER_REQUIRES_ASYNC_CAPTURE
  status)
• Validation: Implemented CreatedPayUponInvoiceOrderValidator to verify PUI
  orders meet PayPal requirements
• Frontend: Added PUI-specific form fields template
  (payUponInvoiceFields.tpl) and payment method icon across PS 1.7, 8.x, and
  9.x

QA Checklist Labels

• Bug fix?
• New feature?
• Improvement?
• Technical debt?
• Covered by tests?

QA Checklist

Additional Context

Frontend Changes

[!] Don't forget to include JIRA task number in PR name or branch name. [!]

[!] Don't forget to update changelog.md with new changes. [!]

• Verify if the version is correct.
• Update demo environments used for testing.
• Ensure changelog contains all changes.
• Update documentation if needed.
• Test if everything works properly.