gh-112328: [Enum] Make some private attributes public. (GH-112514)

* [Enum] Make some private attributes public.

- ``_EnumDict`` --> ``EnumDict``
- ``EnumDict._member_names`` --> ``EnumDict.member_names``
- ``Enum._add_alias_``
- ``Enum._add_value_alias_``

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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Nikita Sobolev <mail@sobolevn.me>