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 using a feature flag. When enabled it shouldn't be much of a change as it assumes that only the top level of StructFields in a struct object are columns. Any structfields that are nested deeper in the schema are instead treated a OBJECT field names. This hierarchy should only exist in structured columns.

[sfc-gh-jrose]

@sfc-gh-aling @sfc-gh-aalam
I've completely refactored the approach to better avoid bcr concerns. I've changed around the logic to more closely match the server-side. Now top level StructFields will be assumed to describe columns as they previously have. Anything nested inside of the schema will instead be considered an actual StructField and have the correct naming logic applied. This approach also gets around the annoying quotation marks that would be added to inner fields.

@sfc-gh-lspiegelberg
I've added you as a reviewer because this change breaks a few AST test and I'm hoping you can explain the process for updating those with the new expectations.

[sfc-gh-aalam]

Looks good. left 2 minor comments

[sfc-gh-aalam]

does is_column=True enforce type of self.column_identifier is ColumnIdentifier? can we add an assert for that in __init__

[sfc-gh-jrose]

__init__ uses the setter for self.name which ensures that self.column_identifier is always ColumnIdentifier.

[sfc-gh-aling]

who's responsible for setting this parameter, also do we need server side parameter for this?

[sfc-gh-jrose]

This should not be enabled until structured type support is rolled out. There will eventually be a server side parameter that we can track instead of this flag.

[sfc-gh-aling]

cool, if this is not rolled out yet then I'm good with this module var