Add end-to-end tests to verify data type support when migrating from MySQL

[nmemond]

This PR adds end-to-end tests to verify data type support when migrating from a MySQL database. It focuses only on the schema migration portion, verification for the data migration is not included here.

The expected data type mappings were determined based on the SMT documentation and Spanner's own documentation.

These end-to-end tests are expected to run against an actual Spanner instance, not an emulated Spanner instance. As such, they have been separated from the other integration tests with a new env var, SPANNER_MIGRATION_TOOL_RUN_E2E_TESTS, to indicate that the E2E tests should be run.

Additionally, given that these run against an actual Spanner instance, they're a bit slower than other tests (on the order of minutes, not seconds). I expect it makes more sense to only run these on-demand instead of for every build.

Also, note that some of these tests don't actually pass: I've written them under the expectation that the mappings defined in the documentation above are correct. Unfortunately, this is not always the case and the documentation differs from the actual implementation for a few of the types. I'll be including the full details of all the issues and disparities in the actual parity report.

• 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 47.22%. Comparing base (6939610) to head (fab27ac).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1243      +/-   ##
==========================================
- Coverage   47.22%   47.22%   -0.01%     
==========================================
  Files         231      231              
  Lines       26698    26698              
  Branches      581      581              
==========================================
- Hits        12609    12607       -2     
- Misses      13368    13370       +2     
  Partials      721      721              

Components	Coverage Δ
backend-apis	42.11% <ø> (ø)
backend-library	51.58% <ø> (ø)
cli	24.44% <ø> (ø)
frontend	38.77% <ø> (-0.05%)	⬇️
see 1 file with indirect coverage changes

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