h2o.set_s3_credentials should support endpoint and region

[hutch3232]

Is your feature request related to a problem? Please describe.
It's convenient that h2o supports changing keys within a session using h2o.set_s3_credentials, but the endpoint and region values are set before launching h2o. It would be nice if those were easily configurable within a session.

Describe the solution you'd like
Two new arguments, perhaps endpoint and region, to h2o.set_s3_credentials. Probably defaulting to NULL or something to not break existing code.

Describe alternatives you've considered
Sparkling water supports this because you can configure bucket-level access which is super nice. That kind of an approach would be even better, if possible.

Additional context
This PR exposed the endpoint/region args: #164 to java