Introduce hint to force specific code set for Code-128 encoding

[gschwaer]

Hi,
This PR enables forcing the code set for the Code128 encoder.
This feature is important when trying to exactly recreate a barcode that has been scanned (see CatimaLoyalty/Android#244).
The problem and the change are described on this post on the Google Group. I ported and tested the change for the master branch.
Thanks for considering this PR.

[srowen]

Looking fine, just some minor comments

[codecov]

Codecov Report

Merging #1411 (e4960d4) into master (ad061f0) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@             Coverage Diff              @@
##             master    #1411      +/-   ##
============================================
- Coverage     79.26%   79.25%   -0.01%     
- Complexity     4367     4382      +15     
============================================
  Files           251      251              
  Lines         14641    14663      +22     
  Branches       3017     3024       +7     
============================================
+ Hits          11605    11621      +16     
- Misses         2152     2154       +2     
- Partials        884      888       +4     

Impacted Files	Coverage Δ
...main/java/com/google/zxing/oned/Code128Writer.java	84.35% <72.00%> (-1.94%)	⬇️
...src/main/java/com/google/zxing/EncodeHintType.java	100.00% <100.00%> (ø)
...om/google/zxing/oned/OneDimensionalCodeWriter.java	77.50% <100.00%> (+0.57%)	⬆️
...om/google/zxing/oned/rss/expanded/ExpandedRow.java	53.84% <0.00%> (-3.30%)	⬇️
...main/java/com/google/zxing/oned/CodaBarReader.java	96.07% <0.00%> (-0.10%)	⬇️
...java/com/google/zxing/aztec/detector/Detector.java	96.44% <0.00%> (+0.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ad061f0...e4960d4. Read the comment docs.