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

Start by tracing Config() and _ENV_CONFIG, then read the follow-up issue #3006 to understand the existing pytest fix and current configuration loading behavior. Done means configuration files and environment variables are no longer read implicitly by Config() or during import, with the intended explicit behavior covered by tests.

Written by the indexing model from the issue text.