Allow passing custom client arguments to S3 client

[shadyvb]

Allows modifying S3 client options via environment variables, so implementations can use options like s3BucketEndpoint to customize the client.

ref https://github.com/humanmade/product-dev/issues/1033 and humanmade/altis-local-server#341

[roborourke]

Looks good, though some client args are nested objects iirc. Do we need more than just whether to do SSL verification and use path style? This does give us some flexibility but might not work for everything.

[shadyvb]

This isn't using path style rather bucket endpoint, so it doesn't play with the URL at all. Traefik then adds the bucket endpoint as a path prefix. Quite the workaround! but fits all our cases.

I wouldn't know what else we'd need, but it made sense to allow arbitrary options for the future. Why not was my philosophy for this change.

[roborourke]

Minor version bump seems appropriate here

[shadyvb]

Released as https://github.com/humanmade/tachyon/releases/tag/v2.6.0

[roborourke]

Nice, docker image should be available by now then