Add end-to-end tests to verify index limits when migrating from PostgreSQL

[nmemond]

This is the second PR to add end-to-end tests to verify table and index limits when migrating from a PostgreSQL database. It focuses only on the schema migration portion, verification for the data migration is not included here.

It adds test for the following index limits:

• indexes per database
• indexes per table
• index name length
• number of columns in an index
• maximum size of an index per row

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 limits are adhered to and that exceeding a limit leads to an error. This is unfortunately not the case for all limits: some succeed without any errors or issues, which of course fails the test. 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 (f703cc9) to head (912f001).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1245   +/-   ##
=======================================
  Coverage   47.22%   47.22%           
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  Files         231      231           
  Lines       26698    26698           
  Branches      581      581           
=======================================
+ Hits        12607    12609    +2     
+ Misses      13370    13368    -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	38.82% <ø> (+0.04%)	⬆️
see 1 file with indirect coverage changes

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