Make it easier to iterate over DictionaryArray

[viirya]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Based on #1263 (comment), seems it is good to have iterator for DictionaryArray so we can easier manipulate it in some cases, e.g. zipping values from two DictionaryArray.

Describe the solution you'd like
A clear and concise description of what you want to happen.

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

Additional context
Add any other context or screenshots about the feature request here.