feat: add stripe payment element support

[mdavo6]

Description

• Adds support for stripe payment element (previous implementation used card element) - see comparison: https://stripe.com/docs/payments/payment-card-element-comparison.
• Order no longer proceeds to complete when payment fails.
• Adds support for error message from payment element when payment fails.
• Adds two new API clients to support new functionality - addPaymentMethod and getPaymentIntent.

Related Issue

#256

Motivation and Context

See description in #256
In short, the Stripe payment element offers 18+ payment methods OOB vs the current implementation which offers card payment only.

How Has This Been Tested?

Tested on chrome. Note: PR will require corresponding spree_gateway PR to work spree/spree_gateway#409.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[mdavo6]

Thanks for the review @letelete . I have updated to address your feedback if you would like to take another look 👍