Add ACH payment processing with saved bank accounts

[rafaelzaleski]

Closes #3787

Changes proposed in this Pull Request:

• Make ACH reusable and introduce tokenization.
• Enhance unit test coverage for ACH.

Testing instructions

Test 1: Add, Remove and Mark the token as default in the my account page.

1. Log in as a customer.
2. Go to My Account > Payment methods.
3. Add a new payment method and select “ACH Direct Debit”
4. After successfully adding the ACH payment method, verify it shows up under Payment methods.
5. Remove the ACH payment method by clicking “Delete” and confirm the token is removed.
6. Add the ACH payment method again and a credit card payment option, then mark ACH as default by selecting “Make default.” Confirm it is set as the default payment method.

Test 2: Save the token at checkout, and use the saved token in a subsequent purchase.

1. Add a product to your cart and proceed to checkout.
2. In the Payment section, choose “ACH Direct Debit.”

• Ensure there’s an option like “Save this payment method”, select it.

3. Enter the required bank details (test routing and account numbers) and complete the checkout.
4. After the order is placed, verify the order confirmation and note that a new ACH token should be created for your account.
5. Start a new purchase (add another product to cart).
6. At checkout, select the saved ACH token.
7. Complete checkout without having to re-enter bank account details. Confirm the purchase is successful and uses the existing ACH token.
8. Repeat for the shortcode and blocks checkout.
9. Go to My Account > Payment methods and confirm the tokens created in this test are present.

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)