Add ignore_case option to snowflake infer schema

[robby-rob-slalom]

Describe the feature

Snowflake supports an IGNORE_CASE option for the INFER_SCHEMA function. This option is not currently passed through in the snowflake__create_external_table macro

Describe alternatives you've considered

Load the tables in as separate sources then union them back together in a later model.

Additional context

This is specific to Snowflake

Who will this benefit?

This is useful for situations when the column case has changed in external files, allowing combined cases to be treated as one column rather than two.