Put the new telemetry submission capability behind a feature

[mhammond]

This is slightly problematic on desktop. There's a good chance a callback interface would work, but that's a little problematic in other ways (as that forces you to use a Box<dyn T> instead of an Arc<>, which is particularly problematic for tests. This seems like a reasonable compromise while we work out a better option.

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[bendk]

This seems like a good workaround for now until we find a better solution for trait interfaces on desktop.