refactor Config() and _ENV_CONFIG

[kevinjqliu]

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

[kevinjqliu]

Should_ENV_CONFIG even exist? its only used in load_catalog and list_catalogs

If we want Config() to be cached, maybe it should be in config.py
We should still make it lazy and not loaded automatically on import

We might want to replace all calls to Config() in the codebase with the cached version, so that we wont need to re-parse everytime