[C#] [NFC] Use semantic model's type where possible

[RReverser]

Description of Changes

This fixes issues with nullability annotations (see the removed TODO), inconsistent field selection between [SpacetimeDB.Type] and [SpacetimeDB.Table], as well as should be more efficient overall as it gets the type model for the structure in one go instead of individually for each field.

API and ABI breaking changes

If this is an API or ABI breaking change, please apply the
corresponding GitHub label.

Expected complexity level and risk

1

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where 1 is a trivial change, and 5 is a deep-reaching and complex change.

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but also to its interactions with existing and future code.

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and what other components it interacts with in potentially concerning ways.

Testing

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• dotnet test
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