Plugin Suggested Quantity. via API call example

[andreievg]

Following up from this PR:, another prototype of adding suggested quantity plugin, that uses API call to get quantities:

• Upload plugin endpoint now accepts json config and url parameter for plugin. name
• Json config from plugin includes common configurations (which hosts are allowed for http requests) and plugin specific configs (in this case url of the server)
• Expose suggested quantity calculation as a plugin (refactor slightly, to only be called once for all item ids being processed)
• Add suggested quantity plugin that queries API for suggested quantities for item (using mock json server)