Add DictionaryArray::key function

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I want to write code to fetch the value of a dictionary array regardless of the type of its key (like apache/datafusion#2721 (comment))

This is definitely possible but it is somewhat awkward today and requires calling to_usize and asserting that that was a valid usize

A way to get the key as usize would make the API easier to use I think -- see
https://github.com/apache/arrow-datafusion/blob/080c32400ddfa2d45b5bebb820184eac8fd5a03a/datafusion/common/src/scalar.rs#L342-L358

for how it has to be done today

Describe the solution you'd like
I would like a way to get the key at index i as a usize

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

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