This adds context to all the relevant cloud provider interface signatures.
Callers of those APIs are currently satisfied using context.TODO().
There will be follow on PRs to push the context through the stack.
For an idea of the full scope of this change please look at PR #58532.
Each major interface is now in its own file. Any package private
functions that are only referenced by a particular module was also moved
to the corresponding file. All common helper functions were moved to
gce_util.go.
This change is a pure movement of code; no semantic changes were made.