[Connect SDK] Integrate with backend calls for client secret

[simond-stripe]

Summary

Adds networking to the example backend (https://stripe-connect-mobile-example-v1.glitch.me/) for getting the publishable key, accounts, and client secret for the SDK.

Motivation

Demonstrates a canonical integration of the ConnectSDK.

Testing

• Added tests
• Modified tests
• Manually verified

Screenshots

TODO

[github-actions]

Risky Change

This is considered a risky change because it adjusts the sample app build.gradle, please review carefully.
We've seen issues in the past which resulted in failed builds for merchants. Please make sure the build.gradle change is intended.

By adding the label accept-risky-change to this PR, I acknowledge that I'm changing an example app and have verified that the SDK remains in a shippable state.