Add S3 source against minio S3 object storage

[mskyttner]

When setting up a connection to an S3 source and not using AWS but for example minio, the change in #1691 allows specifying the "endpoint-url" in source.yaml using a line that says "endpoint: https://my.s3.server.com". This setting cannot be made in the UI, as the S3 settings there does not provide a field for specifying the "endpoint-url".

It would neat to be able to make this setting from the UI and maybe directly from the CLI too, without having to manually edit the source.yaml-file.

Alternatives that I considered was to install and use "aws configure" to set the endpoint URL to use, but due to this long standing bug in the aws CLI, it doesn't work: aws/aws-cli#1270

I would like the "Amazon S3" settings to support also minio (and thereby also other compatible S3 sources), and ideally be renamed "S3" and allow configuration of the endpoint-url.

It would be nice if in addition (ie if not specified) the endpoint URL could be picked up from an environment variable. I also think the "minio client" is nicer to configure rather than the aws CLI if one needs to access data from various different S3 sources (other than just AWS).

[mskyttner]

Related to #1161. "Object storage" which is not distantly located in the outer clouds provide faster latency due to smaller network lags and minio is a component often used to "bring data closer to computations", recently with support also for "legacy" protocols like ftp/sftp.

[mskyttner]

S3 connections against non-AWS servers might be possible with the aws cli if allowing the AWS_ENDPOINT_URL environment variable to be used, see aws/aws-cli#1270 (comment) where this feature recently after 8 years since inception is now available as documented in https://aws.amazon.com/blogs/developer/new-improved-flexibility-when-configuring-endpoint-urls-with-the-aws-sdks-and-tools/.