Improve example uploads plugin entry point

The example upload logic and functionality needs updates. Currently, the example uploads are loaded at app bootup time, which increases the startup time and also means that each individual worker will load them separately.

• Add lazy-loading of example upload resources. The lazy-loading should take place during the start of processing. This requires a new parameter to the uploads API that can be used to indicate which example upload to instantiate.
• Add support for indicating whether to copy the whole directory or it's contents. When specifying a folder (e.g. my_folder or my_folder/, the default should be to copy the entire folder recursively. To copy the contents of a folder recursive, you can use an asterisk (my_folder/*). This behaviour would be consistent with the unix cp and mv commands.
• Add support for mixing online/offline resources
• Add support for specifying multiple resources
• Add helper function (resolve_resource) for handling online/offline files that can be reused in custom load function
• Add Zenodo as an example for where to publish larger tests data
• Update documentation with new interface
• Add an example of a customized load function to the documentation
• Improve tests