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GH-34076: [C#] Allow schema fields with duplicate names (#34125)

Skip fields with duplicate names when building the `_fieldsDictionary` in the Schema constructor.
This allows schemas to have multiple fields with the same name, as the field name is used as the key for this dictionary.

Deprecate the existing `IReadOnlyDictionary<string, Field> Fields` and expose a new `IReadOnlyList<Field> FieldsList` and a new `ILookup<string, Field> FieldsLookup`. Replace usages of `Fields` with `FieldsList` and `FieldsLookup`.
This allows for access to fields that were omitted from the `_fieldsDictionary`.
* Closes: #34076

Lead-authored-by: DanTm99 <danyaal99@hotmail.co.uk>
Co-authored-by: Danyaal Khan <danyaal99@hotmail.co.uk>
Signed-off-by: Weston Pace <weston.pace@gmail.com>