[abstract-csharp] Make data types overridable

[devhl-labs]

Please merge #15806 first as it is a subset of this.

This pr makes the collection of data types overrideable so that we can remove the hard coded ? from the data type in smaller PRs.

@mandrean (2017/08) @frankyjuang (2019/09) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05)

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