[CIVP-24576] FIX relax SQL type checking in `civis_file_to_table`

[jacksonlee-civis]

This pull request relaxes SQL type checking in civis.io.civis_file_to_table, by casting to VARCHAR when type inconsistency is detected for a given column and at least one input file has VARCHAR.

• [x] (For Civis employees only) Reference to a relevant ticket in the pull request title
• [x] Changelog entry added to CHANGELOG.md at the repo's root level
• [x] Description of change in the pull request description
• [x] If applicable, unit tests have been added and/or updated
• [x] The CircleCI builds have all passed

[jacksonlee-civis]

@skomanduri Thank you for catching the column name issue -- this PR is ready for another pass!