Stop automatically generating meta files for assets while using asset processing.

[andriyDev]

Objective

• Today, enabling asset processing can generate many meta files. This makes it a painful transition for users as they get a "mega commit" containing tons of meta files.

Solution

• Stop automatically generating meta files! Users can just leave the meta files defaulted.
• Add a function AssetServer::write_default_meta_file_for_path

Testing

• Tested this manually on the asset_processing example (by removing the meta files for the assets that had default meta files).
• I did not add a unit test for the write_default_meta_file_for_path since we don't have an in-memory asset writer. Writing one could be useful in the future.

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Showcase

Asset processing no longer automatically generates meta files! This makes it much easier to transition to using asset processing since you don't suddenly get many meta files when turning it on.

You can still manually generate meta files using the new AssetServer::write_default_meta_file_for_path function.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[andriyDev]

I don't think this PR needs a migration guide since users don't need to change any existing stuff for their assets to work. New assets will also continue to work, though new meta files will have to be created using the provided function or manually created.

[viridia]

Looks fine to me. Do we want to wait for cart to weigh in on this?

[viridia]

In terms of guides, I can write up something once all changes are in and the dust has settled. Not a migration guide so much as a description of the new workflow.