Make parquet::arrow::array_reader experimental (#1032)

[tustvold]

Which issue does this PR close?

Relates to #1032.

Rationale for this change

I wish to make breaking changes to the APIs in array_reader to address #1132 among others. The APIs are somewhat convoluted to use and so I'm not sure these are intentionally public.

What changes are included in this PR?

This PR makes parquet::arrow::array_reader public only if the experimental feature flag is enabled

Are there any user-facing changes?

This is a breaking change as it removes code from the public API

[codecov-commenter]

Codecov Report

Merging #1133 (03f940e) into master (d852229) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1133   +/-   ##
=======================================
  Coverage   82.33%   82.34%           
=======================================
  Files         169      169           
  Lines       50055    50055           
=======================================
+ Hits        41213    41216    +3     
+ Misses       8842     8839    -3     

Impacted Files	Coverage Δ
arrow/src/array/transform/mod.rs	85.56% <0.00%> (ø)
parquet/src/encodings/encoding.rs	93.71% <0.00%> (+0.19%)	⬆️
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (+0.22%)	⬆️
arrow/src/datatypes/datatype.rs	66.80% <0.00%> (+0.42%)	⬆️

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[alamb]

I think it is a great idea.

@yordan-pavlov any concerns?

[yordan-pavlov]

No concerns from me - I think the APIs in array_reader definitely need some work and I wouldn't be surprised if some breaking changes might be necessary, so a mechanism to allow this (such as what this PR proposes) would be useful; furthermore ArrowArrayReader only uses pub trait ArrayReader (from array_reader).