feat(iframe): add frame messaging for paysg

[KenLSM]

Problem

PaySG aims to integrate FormSG within an IFrame to facilitate payment creation. There's a need for real-time communication from FormSG to PaySG regarding form state changes, such as:

• When a user is in the process of submitting a form
• When a form has been successfully submitted

See #7978 for original PR

Solution

Implement window.parent.postMessage(message, origin) to enable communication from FormSG (child frame) to PaySG (parent window).

To maintain security:

• Implement a whitelist of authorized domains (PaySG domains for different environments)
• Only post messages to whitelisted domains

Implementation Details

1. Create a whitelist of authorized PaySG domains
2. Implement message posting for Storage Forms when form state changes during submission and error
3. Ensure messages are only sent to whitelisted domains

Testing needed to ensure that the message is posted. This is a regression to ensure that Iframe message posting is not removed accidentally.

Breaking Changes

• No - this PR is backwards compatible

Tests

Regression

Payment form can be submitted

• Create a payment form
• As a respondent fill up the payment section
• Ensure that payment form can be successfully submitted