Fixing binary schema migration for mysql to map to correct spanner type with length BYTES(XX)

[darshan-sj]

Fixes #<issue_number_goes_here>

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to README are included in PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.64%. Comparing base (6049111) to head (885e34a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1166      +/-   ##
==========================================
- Coverage   43.65%   43.64%   -0.01%     
==========================================
  Files         227      227              
  Lines       26236    26230       -6     
  Branches      575      575              
==========================================
- Hits        11453    11449       -4     
+ Misses      14051    14049       -2     
  Partials      732      732              

Components	Coverage Δ
backend-apis	44.51% <ø> (ø)
backend-library	46.25% <100.00%> (-0.01%)	⬇️
cli	22.40% <ø> (ø)
frontend	36.19% <ø> (ø)

Files with missing lines	Coverage Δ
sources/mysql/toddl.go	92.81% <100.00%> (+0.98%)	⬆️

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