Fix field names for `ReadFields` and `WriteFields`

[rekhoff]

Fix C# SDK to allow field names of ReadFields and WriteFields, similar to what was done for Read and Write in #2525.

Spawned due to comment #2525 (comment)

Per #2525, regarding piggy-backing off the same implementation method as it used:

Note, however, that some field names still will not work: ReadFields, WriteFields, Equals, and GetHashCode.
This would require a separate fix since the error would happen in a different place. In this case we would need to change the name of the generated member to something like ReadFields_ or Equals_, which I'm not sure is a good idea.

We are not interested in implementing Equals or GetHashCode, but would like to implement the change for ReadFields and WriteFields