Implement resources and resource providers

[bandesz]

Notes

Resources are blocks defining an external stateful resource, e.g. a file or a cloud resource.
Resources always have a unique id, possibly a URI.

Resource providers can get, list, create, update and delete resources.

There should be the following built-in blocks:

• list: generator block which can generate resource block instances, optional filters, can watch
• get: generator block which can load a resource, can watch
• create/update/delete