Backend: implement opaque key format for structures in libraries

[bradenmacdonald]

Based on the discussion in openedx/openedx-learning#282 , implement a new opaque key format for libraries (unless we decide we don't need one).

It should be very similar to LibraryUsageLocatorV2, but perhaps serialized as lc: for library container or ls: for library structure.

[pomegranited]

Implementation should cover:

• Adding the new opaque key type for containers to https://github.com/openedx/opaque-keys
• API method to return the opaque key for a given library unit, like library_component_usage_key.

@bradenmacdonald Should ^ live in new openedx.djangoapps.content_libraries.units.api module? Since there will be several unit-related APIs.

[bradenmacdonald]

@pomegranited I'm not sure if we want a module just for units, because there will also be subsections and sections in libraries in the future. Though now that I think about it, we could put "units" into its own module, or "content" (components + units/subsections/sections) into its own module or "containers" (units/subsections/sections) into its own module, but whatever we do it should just be an internal division of the code, and everything should still be available if you use from openedx.djangoapps.content_libraries import api as library_api