Support infer_schema for CSV and inline file formats in Snowflake

[monti-python]

Solves #254, #284, and #336

Description & motivation

Schema inference (infer_schema: True) doesn't work neither for CSVs nor inline file formats. This is because:

• Inline file formats are not supported with the INFER_SCHEMA() Snowflake function
• CSV file formats need another separate file format for schema inference (one with PARSE_HEADER = True)

This PR solves both issues above and also does:

• Support case-sensitive column aliases by setting ignore_case to False
• Simplify parsing logic
• Harmonize the way columns are treated (declared vs inferred columns)
• Add integration tests for CSV and JSON with schema inference

Checklist

• I have verified that these changes work locally
• I have added an integration test for my fix/feature (if applicable)

[monti-python]

@jeremyyeo @dataders FYI Snowflake integration tests are failing due to authentication errors with your test account.

Tests are running successfully in my own account though.