make has_min_max_set as pub fn

[b41sh]

Which issue does this PR close?

Closes #.

Rationale for this change

in arrow-datafusion#640 we get column max/min value from parquet Statistics. In some cases, if the max/min value is not set, all values are null. When we call max or min method, it will panic. In order to avoid this, we must use has_min_max_set method to check it.

What changes are included in this PR?

make public for fn TypedStatistics::has_min_max_set

Are there any user-facing changes?

• make fn TypedStatistics::has_min_max_set as public API

[codecov-commenter]

Codecov Report

Merging #559 (1c07943) into master (f873d77) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #559   +/-   ##
=======================================
  Coverage   82.47%   82.47%           
=======================================
  Files         167      167           
  Lines       46144    46144           
=======================================
  Hits        38059    38059           
  Misses       8085     8085           

Impacted Files	Coverage Δ
parquet/src/file/statistics.rs	94.62% <100.00%> (ø)

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