feat(schema): support mysql 8

[B4nan]

All ORM tests are now using mysql8, which supports check constraints.
Also fixes several issues with schema diffing in mysql8 (additional queries).

[codecov-commenter]

Codecov Report

Merging #2961 (b8ff889) into master (b513b16) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2961   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          193       193           
  Lines        11806     11817   +11     
  Branches      2725      2726    +1     
=========================================
+ Hits         11806     11817   +11     

Impacted Files	Coverage Δ
packages/core/src/platforms/Platform.ts	100.00% <ø> (ø)
packages/postgresql/src/PostgreSqlPlatform.ts	100.00% <ø> (ø)
packages/sqlite/src/SqlitePlatform.ts	100.00% <ø> (ø)
packages/knex/src/schema/SchemaComparator.ts	100.00% <100.00%> (ø)
packages/mysql/src/MySqlSchemaHelper.ts	100.00% <100.00%> (ø)

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