refactor Config() and _ENV_CONFIG

### Apache Iceberg version

None

### Please describe the bug 🐞

Follow up to #3006, i want to see if we can refactor Config() and _ENV_CONFIG
a couple of issues ive seen
* Config() `__init__` reads both env vars and local config files implicitly, this can be a potentially footgun.
* _ENV_CONFIG is loaded on import time... which calls Config() above and reads env vars and local config files.

#3006 is a fix for pytests. Ideally, I think we can be more explicit here

### Willingness to contribute

- [ ] I can contribute a fix for this bug independently
- [ ] I would be willing to contribute a fix for this bug with guidance from the Iceberg community
- [ ] I cannot contribute a fix for this bug at this time

首先跟踪 Config() 和 _ENV_CONFIG，然后阅读后续 issue #3006，以了解现有的 pytest 修复以及当前的配置加载行为。当配置文件和环境变量不再由 Config() 或在导入期间隐式读取，并且预期的显式行为已由测试覆盖时，即表示完成。

由索引模型根据 Issue 内容生成。