[tokenizer] Add do_lower_case support

[frankfliu]

Description

Brief description of what this PR is about

• If this change is a backward incompatible change, why must this change be made?
• Interesting edge cases to note here

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.31%. Comparing base (ae00167) to head (2f8a234).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3069      +/-   ##
============================================
+ Coverage     72.28%   72.31%   +0.02%     
- Complexity     7302     7314      +12     
============================================
  Files           722      722              
  Lines         32593    32619      +26     
  Branches       3410     3417       +7     
============================================
+ Hits          23561    23588      +27     
- Misses         7407     7408       +1     
+ Partials       1625     1623       -2     

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