[3.0] Miscellaneous generator improvements

[Perksey]

Summary of the PR

• Added a Utf8String type for string constants that collapses down to a string, ReadOnlySpan<byte>, or ReadOnlySpan<sbyte>. This is to alleviate pains with [3.0] Windowing Rewrite #2278 where I was constantly having to MemoryMarshal.Cast these constants to ReadOnlySpan<sbyte>.
• Decoupled the bakery logic from AddApiProfiles. This sets the stage for future use cases of the bakery (e.g. varying struct definitions between runtime-specific source sets for MiniAudio, linked below), and also massively simplifies both mods.
• Change the default INativeContext based implementation of API objects to cache the output of LoadFunction as we did in 2.X. This should massively improve performance.

Related issues, Discord discussions, or proposals

https://discord.com/channels/521092042781229087/521092043288608781/1303461829938249769

Further Comments

#2278 will now be based on this.

[Perksey]

This is ready for review.

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