[Feature] Support multiple HTTP client libraries

[fastjames]

In order to improve flexibility for library users, it would be nice for ex_aws_s3 to support several HTTP client libraries rather than only hackney.

In recent years, a useful pattern has emerged where library authors support multiple HTTP client libraries via shim modules and the use of optional: true in their library mix.exs. Doing this allows application authors to standardize on a single HTTP client library dependency.

I'm willing to create a PR for this change, but I wanted to document it here first in case someone else had additional input (or a ready to go PR!).

[fastjames]

I did some similar work about a year ago on the honeybadger elixir library (honeybadger-io/honeybadger-elixir#579) so I would use that as a reference point.

[fastjames]

After reading over the library code in more detail, it looks like the hackney deps entry is a copy of the one from ex_aws, but hasn't been updated to specify optional: true. I can make that change.