SNOW-1803811: Allow mixed-case field names for struct type columns

[sfc-gh-jrose]

1. Which Jira issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-1803811

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.
   • I acknowledge that I have ensured my changes to be thread-safe. Follow the link for more information: Thread-safe Developer Guidelines

3. Please describe how your code solves the related issue.

   This PR addresses an issue with StructType columns where names for StructFields are always uppercased. Snowflake always uppercases column names, but fields that are internal to StructType columns do not have casing enforced. Ideally we would not treat StructFields as column objects, but separating out that responsibility would require a BCR.

This change gets around the BCR requirement by additionally storing the unmodified version of a field name inside of the ColumnIdentifier class and using that when generating the sql for StructType columns instead of the column formatted name.