fix: support env-var auth config for REST catalog (JSON string decode + flat properties)

[GayathriSrividya]

Summary

Fixes #3422.

This PR implements the fix designed by @kevinjqliu in the issue comment.

RestCatalog._create_session() expected auth to be a dict, but values loaded from environment variables always arrive as strings. This caused auth initialization to fail silently or with a confusing AttributeError for all pluggable auth types (basic, oauth2, google, entra, custom) when configured via PYICEBERG_CATALOG__<NAME>__AUTH.

Fixes

Implementation follows the tolerant auth parsing order suggested by @kevinjqliu:

1. JSON string decode (primary fix)

If the auth property value is a str, JSON-parse it before processing.
This unblocks:

export PYICEBERG_CATALOG__REST__AUTH='{"type":"oauth2", "oauth2":{"client_id":"id","client_secret":"secret","token_url":"https://auth.example/token"}}'

2. Flat env-var property support (alternative fix)

Supports the canonical PyIceberg env-var style — no JSON blob required, no quoting/escaping issues:

export PYICEBERG_CATALOG__REST__AUTH__TYPE=oauth2
export PYICEBERG_CATALOG__REST__AUTH__OAUTH2__CLIENT_ID=id
export PYICEBERG_CATALOG__REST__AUTH__OAUTH2__CLIENT_SECRET=secret
export PYICEBERG_CATALOG__REST__AUTH__OAUTH2__TOKEN_URL=https://auth.example/token

The env-var parser maps these to flat properties (auth.type, auth.oauth2.client-id, etc.). _create_session now checks for auth.type when the auth dict is absent, builds the config dict from the flat auth.* properties, and converts kebab-case keys to snake_case to match AuthManager constructor parameters.

Tests

Five new regression tests added to tests/catalog/test_rest.py (covering the test cases specified by @kevinjqliu):

Test	What it covers
test_rest_catalog_with_basic_auth_as_json_string	JSON string → Basic auth
test_rest_catalog_with_oauth2_auth_as_json_string	JSON string → OAuth2 auth
test_rest_catalog_with_invalid_json_auth_string	Invalid JSON → descriptive ValueError
test_rest_catalog_with_basic_auth_flat_properties	Flat auth.* props → Basic auth
test_rest_catalog_with_oauth2_auth_flat_properties	Flat auth.* props → OAuth2 auth

All 13 test_rest_catalog_with_* tests pass; the 4 pre-existing failures (test_token_200, test_config_200, test_auth_header, etc.) are unrelated to this change and also fail on main.