gh-139398: [Enum] Add supported sunder names to `__dir__` for REPL completions (GH...

gh-139398: [Enum] Add supported sunder names to `__dir__` for REPL completions (GH-139985)

* Add supported sunder names to Enum `__dir__`

This change adds the sunder names `_generate_next_value_`
and `_missing_` to the `__dir__` method of `EnumType` and `Enum`.
In Addition, The instance level sunder names
`_add_alias_` and `_add_value_alias_` are added to `Enum.__dir__`.

With the sunder names exposed in the `dir()` method,
the REPL autocomplete will also show them.

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Co-authored-by: SimonGPrs <107691772+SimonGPrs@users.noreply.github.com>
Co-authored-by: Ethan Furman <ethan@stoneleaf.us>