DEP client uses http.DefaultClient without timeout #50802

Problem

See #3 at #47178

DEP client uses http.DefaultClient if no other client is provided and while Fleet does actually construct it with a fleethttp Client we still don't pass it a timeout anywhere.

Impact

DEP API calls could hang indefinitely if for instance there are hangups on the Apple side

Proposed fix

Initialize DEP client with a fleethttp client with a reasonable timeout everywhere it is initialized. Update the default in the constructor in case any more instantiations are added to our codebase

Evidence

See #3 at #47178