Add `timestamp_format` config to log handers.

We've added a new configuration option `timestamp_format` to the log handlers.
This new config supports the following values:

- `auto`: Automatically determines the timestamp format based on the log formatter being used.
  Utilizes `rfc3339` format for text formatters, and `epoch` format for JSON formatters.
- `epoch`: Represents timestamps in microseconds precision Unix epoch format.
- `rfc3339`: Uses RFC3339 compliant format for date-time strings. For example: `2024-03-26T11:52:19.777087+00:00`.