Generate urls from ex_aws config

[aaronrenner]

This PR makes it so arc returns the default upload url based on what's provided by the ex_aws_config/0 callback. This makes it so arc can be be used with other s3 compatible services like localstack.

The code itself was adapted from ex_aws_s3's presigned url code:

• https://github.com/ex-aws/ex_aws_s3/blob/e21d638f0aebcccb729e138dc680701440bf4980/lib/ex_aws/s3.ex#L1487-L1488
• https://github.com/ex-aws/ex_aws_s3/blob/e21d638f0aebcccb729e138dc680701440bf4980/lib/ex_aws/s3/utils.ex#L296