Add config argument to boto3 resource (closes #168)

[mbarrien]

This pull request adds a config argument to the boto3.resource()/boto3.session.resource() function. This allows passing in a custom config to programatically set a signature version, an s3 addressing style, or any other argument from the botocore client config.

It was done this way over passing a client argument in (as suggested in #168) because it prevents the user from passing in a client of the wrong service type, as illustrated in this hypothetical set of calls.

client = boto3.client('sqs', config=Config())
resource = boto3.resource('ec2', client=client)

(You could make it so that resource ignores the service_name argument passed into it if client is specified, but that's really confusing since service_name is a required argument of boto3.resource().)

The only other non-straightforward thing in this pull request that would be nice to have feedback on is the copy.deepcopy performed if the config has not passed in user_agent_extra. This was done so that the incoming argument is not modified, but another possible semantic is that we use user_agent_extra untouched if a config is passed in, not attempting to override with a default value. Any preference, or okay as is?

[mbarrien]

Ping?

[kyleknap]

I will take a look at it this week. We definitely need a configuration functionality like this in the resource interface. I think accepting the Config object is the way we should go. Another idea that we were considering was the ability to pass in a client to the resource.

[mbarrien]

Note that the description of this pull request above did explicitly discuss a problem with passing a client to the resource. May not block that idea, but it definitely points out how error prone it could be.

[kyleknap]

Those were good points. Seems like there are a lot of edge cases in being able to pass in a client. I think sticking with the config is the right direction for now.

[jamesls]

I agree, 👍 to preferring configs over clients, @mbarrien makes a good case for this.

[jamesls]

if config is not None

[jamesls]

Code looks good to me. Thanks for the pull request!

[mbarrien]

Updated per comment. You're okay with the deepcopy being used here?

[jamesls]

@mbarrien I think that's fine. Thanks for the pull request.