[ShopperInsights] Add EligiblePayments API Call

[scannillo]

Summary of changes

• Update BTAPIClient & BTHTTP to support API calls to paypal.com APIs
• Add BTEligiblePaymentsRequest struct to house POST body for v2/payments/find-eligible-methods API call
• Add relevant unit tests
• Leave TODO for parsing result of API call

Notes

• API call still expected to 403 right now ⚠️
• Adding support for hitting a PayPal API reveals how tightly coupled our networking implementation is to BT API specifics
  • A few highlights:
    • BTHTTP & BTGraphQLHTTP have a lot of overlap & duplication (3 different URLSession configurations, 2 different URLRequest creation sites)
    • No service-agnostic networking interface
  • When time permits, I think there's room to rework things similar to how we did in a series of PPCP PRs
  • Especially if we want to make BraintreeCore the potential unified dependency for networking & analytics b/w PPCP & BT 👀

Checklist

• ~Added a changelog entry~

Authors

@scannillo

[scannillo]

Marking this as "Blocked" since there is talks that the BT GW will wrap this API call, versus us having to hit the PP API directly.

[scannillo]

Removing blocked label, as the API team has updated the API to send mock response values back given certain emails. Follow-up PR to come with parsing logic!