Allow creation of String arrays from &Option<&str> iterators

[koomen]

Which issue does this PR close?

Closes #598.

Rationale for this change

What changes are included in this PR?

Added an implementation of FromIterator<&Option<Ptr>> where Ptr: AsRef<str> for GenericStringArray<OffsetSize>. The new implementation converts iterated values into &str and wraps in an owned Option, then uses the existing FromIterator<Option<Ptr>> implementation to collect into a GenericStringArray.

Are there any user-facing changes?

Users can now create string arrays from &Option<&str> iterators. No breaking changes.

[codecov-commenter]

Codecov Report

Merging #680 (ccb3638) into master (857dbaf) will increase coverage by 0.00%.
The diff coverage is 83.33%.

❗ Current head ccb3638 differs from pull request most recent head 866ced1. Consider uploading reports for the commit 866ced1 to get more accurate results

@@           Coverage Diff           @@
##           master     #680   +/-   ##
=======================================
  Coverage   82.44%   82.44%           
=======================================
  Files         168      168           
  Lines       47326    47347   +21     
=======================================
+ Hits        39016    39034   +18     
- Misses       8310     8313    +3     

Impacted Files	Coverage Δ
parquet/src/column/writer.rs	92.81% <73.68%> (-0.22%)	⬇️
arrow/src/array/array_string.rs	97.88% <100.00%> (+0.06%)	⬆️
arrow/src/ffi.rs	86.54% <0.00%> (-0.27%)	⬇️
parquet/src/encodings/encoding.rs	94.67% <0.00%> (+0.19%)	⬆️

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[alamb]

Thank you very much @koomen this looks awesome. In addition to looking at the code, I also tried out the example from #598 and it works like a charm ❤️

fn main() {
    let string_array: ArrayRef = Arc::new([Some("foo"), None, Some("bar")].into_iter().collect::<StringArray>());
    println!("My awesome string array: {:?}", string_array);
}

Output

My awesome string array: StringArray
[
  "foo",
  null,
  "bar",
]

FYI @nevi-me / @houqp

[koomen]

@alamb Thank you for taking a look! Noob question: Do I need to do anything else to merge this? I'm assuming that someone with permission will do so at some point, but don't want to leave this dangling if it is in fact gating on me :)

[alamb]

Nope no worry @koomen -- nothing is waiting on you. I am waiting to see if any of the other maintainers wants to comment and if not I plan to merge it in later today or tomorrow.

Also, FWIW I plan to backport this for inclusion in 5.2.0 (eta release early next week)

[jorgecarleitao]

LGTM. Thanks for your contribution!

[alamb]

Given I plan to make a 5.2.0 release candidate today I took the liberty of applying @jorgecarleitao 's suggestions to add links to the doc comments to this PR and will merge when it passes CI. cc @koomen

Thanks again