Add DictionaryArray::key function

[alamb]

Which issue does this PR close?

Closes #1911

Rationale for this change

See #1911

What changes are included in this PR?

1. Add DictionaryArray::key function
2. Tests

Are there any user-facing changes?

Yes new function

[codecov-commenter]

Codecov Report

Merging #1912 (dcdbc50) into master (ded6316) will decrease coverage by 0.00%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master    #1912      +/-   ##
==========================================
- Coverage   83.41%   83.41%   -0.01%     
==========================================
  Files         214      214              
  Lines       56991    57004      +13     
==========================================
+ Hits        47541    47550       +9     
- Misses       9450     9454       +4     

Impacted Files	Coverage Δ
arrow/src/array/array_dictionary.rs	91.53% <84.61%> (-0.39%)	⬇️
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.23%)	⬇️
parquet/src/encodings/encoding.rs	93.43% <0.00%> (-0.20%)	⬇️

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[waynexia]

This is a nice interface 👍 I left a comment about that expect.

[waynexia]

For most cases this unwrap won't panic, but do we need to maintain the same behavior with https://github.com/apache/arrow-datafusion/blob/080c32400ddfa2d45b5bebb820184eac8fd5a03a/datafusion/common/src/scalar.rs#L342-L358 ? If so the return type can either be Result<Option<_>> or Option<_>, I'm ok with both (hard to choose...).

[tustvold]

Panicking should be fine, it's a validation failure if the array contains negative indexes. TBH I keep meaning to change all these checked conversions to numeric casts (i.e. as), I wouldn't be surprised if this leads to non-trivial performance benefits.

[alamb]

Filed #1918 to track

[viirya]

Looks good to me. Currently to I need to call value on keys array. This should be convenient.