Support multple configurations for S3

[achedeuzot]

Environment

• Elixir version (elixir -v): 1.11.4
• Waffle version (mix deps): 1.1.4
• Waffle dependencies when applicable (mix deps): waffle_ecto 0.0.10
• Operating system: macOS 11.3.1

Expected behavior

I'd like for the current S3 storage driver to allow multiple S3 configurations (copy files from one bucket to another).

Actual behavior

The S3 storage uses the ExAws.S3 default authentication system.

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I'll try to provide an updated S3 storage with this new possibility ;)