Add support for progress_callback in Object#download_file

[alextwoods]

Implements #2901
Closes #2681

This adds support for a progress_callback option in Object#download_file, similar to what is supported in upload_file.

This also improves the performance of download_file when multiple threads are used - previously it was done in batches, requiring all threads in the batch to complete before moving onto another batch (and additionally, creating a new set of threads each time instead of re-using one set of worker threads). This follows the approach used in the MultipartUploader.

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1. To make sure we include your contribution in the release notes, please make sure to add description entry for your changes in the "unreleased changes" section of the CHANGELOG.md file (at corresponding gem). For the description entry, please make sure it lives in one line and starts with Feature or Issue in the correct format.

2. For generated code changes, please checkout below instructions first:
   https://github.com/aws/aws-sdk-ruby/blob/version-3/CONTRIBUTING.md

Thank you for your contribution!

[mullermp]

Seems fine