Remove Futures / Promise based API client in favor of async client

[mats-stripe]

Context: https://docs.google.com/document/d/1q8sSaEKv0AO_RXwpgbhtAkGL77NadeMf-Ab-UJuNa5U/edit?usp=sharing

Summary

This removes the FinancialConnectionsAPIClient (Futures / Promise based) in favor of the FinancialConnectionsAsyncAPIClient. Here's where this leaves us:

• Our only API client is powered by Swift Concurrency.
• The API client conforms to the Futures / Promise based protocol.
• All call sites (view controllers / data sources) interface with a Futures / Promise based API, as they already do today.

With that:

• We only maintain one API client going forward.
• We can still migrate all call sites to a Swift Concurrency based API if we chose to.

Motivation

Cleanup our in progress migration to Swift Concurrency.

Testing

All of our E2E test have already been using the Async API client! I also added more tests for the async poller in this PR.

Changelog

N/a

[github-actions]

🚨 New dead code detected in this PR:

APIPollingHelper.swift:11 warning: Class 'APIPollingHelper' is unused

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