Add utc_tz_aware="schema" mode for schema-matching datetime output

[joe-clickhouse]

Summary

• Adds utc_tz_aware="schema" mode which returns datetimes that match the server's explicit column definition. That means it'll be timezone-aware when the column schema defines a timezone e.g. DateTime('UTC') and naive for bare DateTime columns
• Validates the utc_tz_aware parameter and raises ProgrammingError for unrecognized string values
• Logs a warning when "schema" mode is used with Arrow-based query methods as they don't yet support this mode since ClickHouse attaches the server timezone to all columns in Arrow format

Note on naming

Extending utc_tz_aware from a boolean to also accept a string is admittedly not the cleanest API. The parameter name implies UTC-specific behavior, but "schema" mode applies to all timezones. This is intentional as a short-term unblock for users who need schema-matching behavior now. I'll open a separate issue to rename utc_tz_aware to something more appropriate like tz_mode as part of the 1.0 release which will be happening soon.

Closes #645

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG

[mitchhollander]

Commented with a take it or leave it suggestion.

[joe-clickhouse]

Just to follow up here, I've pushed a commit to make these changes. You can see them on the branch itself, but for some reason GitHub hasn't propagated to here yet. I'll let this sit for a bit and see if it eventually resolves.

EDIT: pushed empty commit to trigger PR sync and it worked.