Add `as_decimal_array(arr: &dyn Array) -> &DecimalArray`

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
There are many as_XXX functions such as as_boolean_array to conveniently downcast arrays to the appropriate concrete Array types, but not one for DecimalArray 😢 .

The full list of supported ones is in the doc https://docs.rs/arrow/9.0.2/arrow/array/index.html#functions)

Describe the solution you'd like
I would like one for DecimalArray:

fn as_decimal_array(arr: &dyn Array) -> &DecimalArray {
...
}

Additional context
Context: I found while working on apache/datafusion#1475

[alamb]

cc @liukun4515

[liukun4515]

cc @liukun4515

If no one takes this, I am glad to do this task.
@alamb Please assign it to me.

[liukun4515]

I will do this on the weekend, and focus on some knowledge about ballista and distributed scheduler recently.