Make GatedReader a test-only symbol, and allow all bevy_asset tests to all run single threaded.

[andriyDev]

Objective

• Some tests have been disabled in single-threaded mode. Now they aren't!

Solution

• The solution to allowing all tests to run single threaded is to make GatedReader await for gates to open rather than blocking on it.
• This however requires adding async-channel (already in tree). To avoid this additional dependency in most cases, I made this a dev-dependency.
• However, this now means that GatedReader can only be used in tests (in bevy_asset). We could make this a feature flag, but it seems highly unlikely that real users need a GatedReader - and if they need it for their test, they can just fork the GatedReader.

Testing

• The bevy_asset tests pass without setting the --features multi_threaded flag!

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Migration Guide

bevy_asset::io::gated::GatedReader and bevy_asset::io::gated::GatedOpener are no longer accessible to users.