New: Extend _access with users and groups keys

[taylortom]

Context

Part of the generic _access mechanism — see adapt-security/adapt-authoring-api#98.

Summary

Refactor the existing group-based sharing to use the new _access structure:

• Create a schema extension that adds _access.users (array of user IDs) and _access.groups (array of group IDs) to registered modules
• Update registerGroupModule (or rename to registerSharingModule) to extend with the new schema and tap accessCheckHook for both _access.users and _access.groups checks
• Replace the current userGroups top-level field with _access.groups
• Replace the current usergroups.schema.json extension with the new _access-based extension
• Update hasGroupAccess to read from resource._access.groups
• Add hasUserAccess utility for _access.users check

Dependencies

• Requires New: Add generic _access mechanism to AbstractApiModule adapt-authoring-api#98 to land first