feat: support dynamic timestamp ScanLocation via DSN timezone

[almas1992]

• Do only one thing
• Non breaking API changes
• Tested

Background

By default, pgx parses PostgreSQL timestamp (without time zone) fields as UTC.
However, in many real-world scenarios, timestamps are stored in local time (e.g., Asia/Shanghai).

To address this, the timezone parameter is now extracted from the DSN and used to set
the ScanLocation of the pgtype.TimestampCodec, ensuring correct interpretation of timestamp values.

What did this pull request do?

• Parses the timezone value from the DSN, if provided
• Loads the corresponding Go *time.Location
• Registers a custom TimestampCodec with ScanLocation set to the parsed location

This ensures that timestamp parsing behavior is aligned with the configured session time zone.

User Case Description

In many PostgreSQL schemas, timestamp fields (without time zone) are used to store datetime values in local time, such as Asia/Shanghai.
However, by default, the pgx driver parses these values as UTC, which can lead to incorrect time interpretation in applications — for example, an event saved as 2024-01-01 08:00:00 may be misinterpreted as 2024-01-01 08:00:00 UTC instead of Asia/Shanghai.

This becomes problematic when:

• Displaying times to end-users expecting local time
• Comparing or calculating time ranges across services
• Debugging issues where time shifts are hard to trace

To solve this, the DSN now accepts a timezone parameter (e.g., timezone=Asia/Shanghai), which is used to configure pgx’s TimestampCodec.ScanLocation.
This ensures that timestamp values are interpreted in the correct local time zone, aligning with how the data was originally stored.

This change improves consistency, prevents subtle bugs, and reduces the need for manual .In(time.Local) conversions in application code.