Add option and environment variable to specify S3 endpoint URL

[jdidion]

This addresses #14. It adds a command line option to the uploader (--s3endpoint) to specify an alternative endpoint, and defaults to the value of an environment variable (MINIWDL__AWS__S3_ENDPOINT_URL). If specified, the endpoint is specified as the value of the aws CLI --endpoint-url option.

[mlin]

LGTM modulo running the formatter (pre-commit run --all-files). I think this will work with AGC by adding ---s3endpoint to engineOptions in the workflow MANIFEST.json (but we won't know for sure until trying it at some undetermined future point when they pull in a new version)