exclude column from schema validation

Sometimes users run into a case where the schema validation is too strict, and their program actually works as is; it would be really nice to given them a way to exclude a column from validation. Otherwise, the developer of the schema validation tool has to make some fairly arbitrary calls about how strict to be and about what to tolerate and what not to tolerate.

The issue names no files, tests, or entry points. Start by locating the schema-validation implementation and existing column-mapping tests, then determine how an excluded column should be configured and validated. Done should include documented behavior and tests showing that the selected column is skipped without weakening validation for other columns.

Written by the indexing model from the issue text.