refactor: explicit Config.load() + lazy catalog config initialization

[jx2lee]

Closes #3028

Rationale for this change

This PR refactors config loading to avoid implicit IO and import-time config initialization.

Changes

• Make Config() a pure container (no implicit file/env reads)
• Add explicit Config.load() to load from config files + environment variables
• Replace import-time _ENV_CONFIG with a lazy cached getter in pyiceberg.catalog
• Update affected call sites to use Config.load() where runtime config reads are intended

Are these changes tested?

Yes!

Are there any user-facing changes?

No! (Maybe.. ?)

[jx2lee]

@kevinjqliu
I took a pass at a refactoring (a draft 🤗 ) based on your two concerns:

1. Config() implicit IO (env + local config files)
2. import-time _ENV_CONFIG initialization causing config reads during import

---

Direction I used is:

• make Config() a pure container (no implicit)
• add explicit Config.load() for file/env loading
• replace import-time _ENV_CONFIG with a lazy cached getter used by load_catalog() / list_catalogs()

---

Would you mind taking a quick look and sharing any guidance on:

• whether changing Config() semantics so config loading is explicit (e.g., via Config.load()) is acceptable in scope for this issue
• any compatibility concerns I should watch for before opening a PR

Thanks — happy to adjust the approach based on your feedback. 👍🏽