Support AWS S3 Express One Zone

[4141done]

Is your feature request related to a problem? Please describe.
The S3 Gateway should provide support for AWS S3 Express One Zone

Describe the solution you'd like
Users should be able to attach the gateway to AWS S3 Express One Zone or normal s3-compatible object stores with roughly the same configuration if possible

Additional context
Relevant discussion: #220

[hveiga]

Hi @4141done I opened #228 to add support for S3 Express. After some research I found out the missing pieces and how to make them work. Looking for feedback on the approach, as I believe it can be improved. Thanks.

[4141done]

Thanks to @hveiga 's contribution we were able to implement compatibility #229

For usage instructions see this section of the readme: https://github.com/nginxinc/nginx-s3-gateway/blob/master/docs/getting_started.md#user-content-choosing-a-s3_style-setting

The session-based auth is not implemented. I've added an issue for it here #233