ref(service): Make object key optional on insert

[jan-auer]

Changes the object signature and makes keys optional. This allows us to add an
optimization that skips checking for tombstones when server-assigned keys are
used.

This PR introduces a new type ObjectContext. It stores the usecase and scopes
of the object's ID. The ObjectId type now wraps a context and has convenience
constructors and getters for all its parts. This type can be used by auth
instead of the ObjectId, since the key part is not needed. This will further
allow us to provide auth on batch endpoints and collection operations.

The StorageService interface now has an insert_object that replaces the
former put_object. This method takes an optional key and internally creates
a random ID if needed.

[lcian]

Nice change!