[csharp-netore][generichost] Renamed ToModel to AsModel

[devhl-labs]

When you cast an object using the as keyword, the object is nullable such as TType?
This change mirrors that convention, and also allows us to have a ToModel which returns TType or throws. That will come in a later PR.

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[wing328]

@devhl-labs thanks for the PR, which has been merged. I've marked it as a breaking change without fallback.