[C#] Make FieldDescriptor propertyName public

[philjdf]

Saw in #4120 that this small change was on the cards. It gives a clean way to go from C# property names to ProtoBuf snake_case names.

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[philjdf]

@googlebot I signed it!

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[philjdf]

@googlebot I fixed it.

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[deannagarcia]

Thanks for this contribution and sorry for the delay, merging now!