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GH-33970: [C#] Make schema field names case sensitive (#33978)

Remove the use of a comparer that ignores casing when building the fields dictionary for a schema. 

This allows for fields with names that differ only in casing.

Currently, fields with names that only differ in casing will be seen as having the same name and will cause an Exception to be thrown due to being seen as having duplicate keys.
* Closes: #33970

Authored-by: Danyaal Khan <danyaal99@hotmail.co.uk>
Signed-off-by: Eric Erhardt <eric.erhardt@microsoft.com>