Add get_bit to BooleanBufferBuilder

[boazberman]

Which issue does this PR close?

Ref apache/datafusion#342
Ref apache/datafusion#240

Rationale for this change

Described in the DataFusion issue. To make BooleanBufferBuilder useable as a bit vector within DataFusion code I needed this api exposed as well.

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #693 (eb2ff69) into master (1a5c26b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #693   +/-   ##
=======================================
  Coverage   82.44%   82.44%           
=======================================
  Files         168      168           
  Lines       47347    47365   +18     
=======================================
+ Hits        39033    39052   +19     
+ Misses       8314     8313    -1     

Impacted Files	Coverage Δ
arrow/src/array/builder.rs	86.13% <100.00%> (+0.15%)	⬆️
parquet/src/encodings/encoding.rs	94.67% <0.00%> (+0.19%)	⬆️

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[Dandandan]

👍

[alamb]

Looks good to me. The tests look good as well

cc @jorgecarleitao @nevi-me

[boazberman]

How many approvers does one need to get a pr merged?

[Dandandan]

How many approvers does one need to get a pr merged?

We don't have a minimum reviewers requirement. The PRs will be merged after some common sense judgement, based on the complexity of the changes and giving other reviewers time to review and comment.

As the change is small and not really controversial I just merged it
Thanks again! 🎉

[alamb]

I plan to backport this change to 5.3.0 (eta in about two weeks time)

[alamb]

Backport in #699. I plan to make 5.3.0 release candidate around Aug 27