Add more load_direct implementations

[ricky26]

Objective

• Introduce variants of LoadContext::load_direct which allow picking asset type & configuring settings.
• Fixes #12963.

Solution

• Implements ErasedLoadedAsset::downcast and adds some accessors to LoadedAsset<A>.
• Changes load_direct/load_direct_with_reader to be typed, and introduces load_direct_untyped/load_direct_untyped_with_reader.
• Introduces load_direct_with_settings and load_direct_with_reader_and_settings.

Testing

• I've run cargo test and played with the examples which use load_direct.
• I also extended the asset_processing example to use the new typed version of load_direct and use load_direct_with_settings.

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Changelog

• Introduced new load_direct methods in LoadContext to allow specifying type & settings

Migration Guide

• LoadContext::load_direct has been renamed to LoadContext::load_direct_untyped. You may find the new load_direct is more appropriate for your use case (and the migration may only be moving one type parameter).
• LoadContext::load_direct_with_reader has been renamed to LoadContext::load_direct_untyped_with_reader.

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This might not be an obvious win as a solution because it introduces quite a few new load_direct alternatives - but it does follow the existing pattern pretty well. I'm very open to alternatives. :sweat_smile:

[ricky26]

@alice-i-cecile I've had a go at implementing a builder-style API. I suspect this is more open to bike-shedding (in particular .load_direct().load() isn't my favourite), and more likely to have other issues. However, I did the simpler tidy-up in the prior commits, in case this proves to be more controversial.

[alice-i-cecile]

Hmm, I don't think that's an improvement 😅 Neat to see, but I would probably revert the builder experiment. I'll ask folks for more opinions / ideas.