Optionally allow HTTP redirects

[simontraill]

This adds (optional, client configured) support for the HTTP "location" header when processing HTTP redirects for S3 responses.

I'm a developer at NVIDIA; I wrote this on behalf of the AIStore project. We'd really like to use botocore (and boto3) for client access; AIStore (and some other systems - like Apache Ozone, referenced by this issue) rely on standard HTTP redirects - using the "location" header - for load balancing.

botocore constructs redirection URLs using the region instead, and won't allow redirections outside of known Amazon URIs.

I've gated the change here behind a config setting; that said, other AWS SDKs (aws-sdk-go, for instance) do appear to support redirects as normal, so hopefully this behaviour is in line with other clients.

This change -

• Introduces allow_http_redirects config option (default: False).
• If set to True, performs HTTP redirection based on the Location header
• Limits the number of redirections for a given request to three (I didn't feel adding another config option was worth it in this case).
• Closes Support S3 HTTP redirects to non-Amazon URI's by using the location header #2571.

import boto3
from botocore.config import Config as Config

config = Config(allow_http_redirects=True)
session = boto3.Session()
s3 = session.resource("s3", endpoint_url="http://127.0.0.1:8080/s3", config=config)

I've added unit and functional tests and tried to keep the style consistent with your existing work. If there's any demand for caching using the Cache-Control or Expires headers, I could certainly do it in the follow-up.

Thanks.