Prevent partial application of Envoy extensions
Ensure that non-required extensions do not change xDS resources before
exiting on failure by cloning proto messages prior to applying each
extension.
To support this change, also move `CanApply` checks up a layer and make
them prior to attempting extension application, s.t. we avoid
unnecessary copies where extensions can't be applied.
Last, ensure that we do not allow panics from `CanApply` or `Extend`
checks to escape the attempted extension application.
Michael Zalimeni