Add validation for SDK token when createSubscription is being passed

[9zees]

Description

When the Unbranded CardFields is used for Subscriptions, we require an SDK token data-attribute to be passed, similar to 3DS Component. This PR adds a validation for this token when a createSubscription callback has been passed. This validation should not be invoked when a createSubscription callback is not passed.

Acceptance Criteria:

1. A validation error is thrown when attempting to use Card Fields subscriptions without the SDK token.
2. A validation error is not thrown when attempting to use Card Fields without subscriptions when an SDK token is not provided.

NOTE: I was intending on using Zoid's validate() method but not able to get it invoked despite of following the documentation. Leaving this here as proof that I did attempt to use this method.

There are 2 ways we can validate on Zoid.
(1) Use validate() function in zoid.create(...)
https://krakenjs.com/zoid/docs/api/create.html#validate--props---void

(2) Use validate() function in individual props
https://krakenjs.com/zoid/docs/api/prop-definitions.html#validate

I'm choosing the prop-level validation because we only want to check for createSubscription (prop-level) as opposed to running the validate method each time on all the props.

=================== End of Note ====================

Why are we making these changes? Include references to any related Jira tasks or GitHub Issues

https://paypal.atlassian.net/browse/DTPPCPSDK-2628

Reproduction Steps (if applicable)

Screenshots (if applicable)

Use-Case 1: When we pass in SdkToken and createSubscription:

Use-Case 2: When we don't pass SdkToken but pass createSubscription:

Use-Case 3: When we don't pass SdkToken and don't pass createSubscription (but pass createOrder instead) :

Dependent Changes (if applicable)

Groups who should review (if applicable)

❤️ Thank you!