improve utf8 init validity

[ritchie46]

The utf8 version did still create an initial validity bitmap from a chained iterator instead of the more performance extend_constant. This PR changes that so that it aligns with the other arrays.

I also allocate the initial size of MutableBitmap to the capacity of the data array. If a user has intialized the structs with_capacity this information is already known.

[codecov]

Codecov Report

Merging #530 (8a8cde6) into main (2e55314) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #530      +/-   ##
==========================================
- Coverage   80.20%   80.20%   -0.01%     
==========================================
  Files         381      381              
  Lines       23730    23730              
==========================================
- Hits        19033    19032       -1     
- Misses       4697     4698       +1     

Impacted Files	Coverage Δ
src/array/boolean/mutable.rs	82.68% <100.00%> (ø)
src/array/list/mutable.rs	66.33% <100.00%> (ø)
src/array/primitive/mutable.rs	83.79% <100.00%> (ø)
src/array/utf8/mutable.rs	85.25% <100.00%> (ø)
src/io/avro/read/schema.rs	54.76% <0.00%> (-1.20%)	⬇️

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[houqp]

The integration test is failing probably due to new unrelated changes in upstream arrow repo.