[Feature] Support virtual host S3 API calls #52

After struggling for several hours with Docker Compose, I noticed my problem was actually in this library, which does not seem to support the virtual host addressing for the s3 client.

Looking at boto3 in debug mode, I noticed that when using Virtual host addressing, the HTTP request was aimed towards http://<bucket>.<endpoint_url>:<endpoint_port>. This is not a valid endpoint, and it should be http://<bucket>.s3.<endpoint_url>:<endpoint_port>, as described here.

After delving deeper I noticed localstack_client.config.get_endpoint it is not handling s3 in any special way. I manually patched the function and noticed that with a simple

def new_get_service_endpoint(
        service: str, localstack_host: Optional[str] = None
    ) -> Optional[str]:
        endpoints = localstack_client.config.get_service_endpoints(localstack_host=localstack_host)
        endpoint = endpoints.get(service)
        if service == "s3":
            endpoint = "http://s3." + endpoint.split("http://")[1]
        return endpoint

I know this doesn't handle SSL, it's just a draft. It works for both virtual and path based addressing.

I see three ways simple about this:

1. Do not support virtual host addressing. This is a problem, as in theory AWS is deprecating path based addressing (also it has been deprecating it for 4 years, so...)
2. Do a hack like the one proposed.
3. Honor AWS service specific endpoints through envars (for instance AWS_ENDPOINT_URL_S3) and leave it up to the user to set it up (with appropriate documentation, at least in the README.md).

What are your thoughts on this?