[V7] Add Encodable protocol for PayPal Vault

[richherrera]

Summary of changes

• Add encodable types for: BTPostalAddress, BTPayPalBillingCycle, BTPayPalBillingPricing, BTPayPalRecurringBillingDetails, BTPayPalRecurringBillingPlanType and BTPayPalLineItem.
• Remove unnecessary BTPayPalRequest concrete class.
• Move the UTs from the concrete class BTPayPalRequest to the classes BTPayPalCheckout and BTPayPalVault since both classes previously inherited from it.
• A method encodedPostBodyWith was added to the PayPalRequest interface to create POSTBody models.
• Rename the PayPalRequest protocol to BTPayPalRequest.
• Since the ExperienceProfile model is essentially the same in both the Checkout and Vault flows, I created the PayPalExperienceProfile model.
• Add encodable types for post bodies used in the PayPal Vault flow.
• PayPalVault has been tested in the Demo app to ensure the dictionaries are still constructed as expected.

Checklist

• [ ] Added a changelog entry
• Tested and confirmed payment flows affected by this change are functioning as expected

Authors

• @richherrera

[jaxdesmarais]

👏 this is super clean, nice work on this!

[jaxdesmarais]

Super nit, can we put this optional after the required order wise?