Default data source timeout is not configurable

[zillaer]

What happened:
通过数据源执行的sql超过30s超时，数据源配置中无超时时间修改配置项

What you expected to happen:
数据源配置中增加超时时间配置项

How to reproduce it (as minimally and precisely as possible):

Screenshots

greptimedb前端nginx返回499，grafana数据源返回502，代表数据源主动断开

Anything else we need to know?:

Environment:

• Grafana version:
• Plugin version:
• OS Grafana is installed on:
• User OS & Browser:
• Others: