Public kernel does not verify global variables injected into public context

[spalladino]

The PublicContextInputs injected into a public function include a set of PublicGlobalVariables (such as coinbase, block number, timestamp, etc). However, these are not returned in the PublicCircuitPublicInputs created by the PublicContext.finish(), so the public kernel has no visibility of them, so a malicious sequencer could inject whatever value they want into those variables.