[csharp-netcore] Add option skip generating getter for sub-schemas

[wing328]

• add option skip generating getter for sub-schemas to avoid duplicated method names (getter)
• fix typo

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

[devhl-labs]

What scenario causes duplicate method names? I think the way oneof/anyof is handled in this library is odd, but that's neither here nor there.