Move DownloadManager to PaymentSheet

[davidme-stripe]

Summary

This contains async/await code, which was causing StripeApplePay to pull in the Swift Concurrency libraries on iOS 14.0 or earlier. Move it to StripePaymentSheet, as it isn't used by other frameworks.

Motivation

#3548

Testing

swift-package-info binary-size --local-path . --product StripeApplePay

Changelog

Will add

[davidme-stripe]

+-----------------------------------------------+
|              Swift Package Info               |
|                                               |
|           StripeApplePay, undefined           |
+-------------+---------------------------------+
| Provider    | Results                         |
+-------------+---------------------------------+
| Binary Size | Binary size increases by 1.2 MB |
+-------------+---------------------------------+
> Total of 1 provider used.

[emerge-tools]

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