fix: specify VARCHAR size when loading examples

[betodealmeida]

SUMMARY

superset load_examples is failing on Mysql:

  File "/Users/max/.pyenv/versions/3.7.7/envs/env37/lib/python3.7/site-packages/sqlalchemy/sql/elements.py", line 481, in compile
    return self._compiler(dialect, bind=bind, **kw)
  File "/Users/max/.pyenv/versions/3.7.7/envs/env37/lib/python3.7/site-packages/sqlalchemy/sql/ddl.py", line 29, in _compiler
    return dialect.ddl_compiler(dialect, self, **kw)
  File "/Users/max/.pyenv/versions/3.7.7/envs/env37/lib/python3.7/site-packages/sqlalchemy/sql/compiler.py", line 322, in __init__
    self.string = self.process(self.statement, **compile_kwargs)
  File "/Users/max/.pyenv/versions/3.7.7/envs/env37/lib/python3.7/site-packages/sqlalchemy/sql/compiler.py", line 352, in process
    return obj._compiler_dispatch(self, **kwargs)
  File "/Users/max/.pyenv/versions/3.7.7/envs/env37/lib/python3.7/site-packages/sqlalchemy/sql/visitors.py", line 96, in _compiler_dispatch
    return meth(self, **kw)
  File "/Users/max/.pyenv/versions/3.7.7/envs/env37/lib/python3.7/site-packages/sqlalchemy/sql/compiler.py", line 2918, in visit_create_table
    from_=ce,
  File "/Users/max/.pyenv/versions/3.7.7/envs/env37/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
sqlalchemy.exc.CompileError: (in table 'video_game_sales', column 'Name'): VARCHAR requires a length on dialect mysql

This PR fixes it by adding a default size for VARCHAR columns.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TEST PLAN

superset load_examples now work on MySQL.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #12186 (8fe9dd2) into master (9b110b2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #12186   +/-   ##
=======================================
  Coverage   63.07%   63.07%           
=======================================
  Files         994      994           
  Lines       49065    49065           
  Branches     4982     4982           
=======================================
  Hits        30946    30946           
  Misses      17919    17919           
  Partials      200      200           

Flag	Coverage Δ
javascript	61.23% <ø> (ø)
python	64.25% <ø> (ø)

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Impacted Files	Coverage Δ
superset/datasets/commands/importers/v1/utils.py	58.62% <ø> (ø)

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