Add TimeZone setting in OpenMLDB

**Is your feature request related to a problem? Please describe.**

Now, OpenMLDB only supports UTC/GMT+08:00. Users feel frustrate when they are trying to convert the time string to the timestamp in other time zones.

In this issue, we are going to support the time zone setting from OpenMLDB.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.

The issue mentions adding timezone support to OpenMLDB, which currently only supports UTC/GMT+08:00. Look for time-related functions and timestamp handling in the codebase, likely in SQL parsing or date/time modules. Check existing documentation or tests for current timezone behavior to understand the scope. Determine how timezone settings should be configured (e.g., session variable, global setting) and implement accordingly.

Scritto dal modello di indicizzazione a partire dal testo della issue.