Provide an s3 compatible REST api for ozone objects

S3 REST api is the de facto standard for object stores. Many external tools already support it.

This issue is about creating a new s3gateway component which implements (most part of) the s3 API using the internal RPC calls.

Some part of the implementation is very straightforward: we need a new service with usual REST stack and we need to implement the most commont GET/POST/PUT calls. Some other (Authorization, multi-part upload) are more tricky.

Here I suggest to create an evaluation: first we can implement a skeleton service which could support read only requests without authorization and we can define proper specification for the upload part / authorization during the work.

As of now the gatway service could be a new standalone application (eg. ozone s3g start) later we can modify it to work as s DatanodePlugin similar to the existing object store plugin.