Bug: REST catalog auth cannot be configured via environment variables unless auth JSON strings are decoded

### Apache Iceberg version

None

### Please describe the bug 🐞

### Summary
`RestCatalog._create_session()` expects `auth` to be a dict. When catalog config comes from environment variables, values are strings, so `auth` is received as a string and auth initialization fails.

This blocks env-var-based configuration for pluggable REST auth (`basic`, `oauth2`, `google`, `entra`, `custom`) unless string JSON is explicitly decoded first.

### Minimal repro
```bash
export PYICEBERG_CATALOG__REST__TYPE=rest
export PYICEBERG_CATALOG__REST__URI=http://localhost:8181
export PYICEBERG_CATALOG__REST__AUTH='{"type":"oauth2","oauth2":{"client_id":"id","client_secret":"secret","token_url":"https://auth.example/token"}}'
```

```python
from pyiceberg.catalog import load_catalog
load_catalog("rest")
```

### Actual (without this fix)
Expected: catalog initializes and uses the configured auth manager.

Actual: initialization fails because `auth` is treated as a string and `.get(...)` is called on it.

### Suggested fix
In REST catalog session setup, if `auth` is a string, decode it as JSON before reading `auth.type` and type-specific config.

Add regression tests for both:
- `PYICEBERG_CATALOG____AUTH` (JSON string) initializes auth manager correctly.
- `PYICEBERG_CATALOG____AUTH__...` maps correctly into auth manager configuration.

### Alternative fix (follows current env-var standard)
Support flattened auth properties from environment variables instead of requiring a JSON blob in `...__AUTH`.

Example:

```bash
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
```

This aligns with existing flattened env-var configuration behavior and avoids JSON-in-env quoting/escaping issues.

### Verification
Observed with current code path (`Config._from_environment_variables`):

```bash
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
```

Parsed result:

```python
{'catalog': {'rest': {'auth.type': 'oauth2', 'auth.oauth2.client-id': 'id', 'auth.oauth2.client-secret': 'secret'}}}
```

This confirms flattened `AUTH__...` env vars are currently stored as dotted keys, not as a nested `auth` object consumed by `RestCatalog._create_session()`.

### 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

先閱讀 RestCatalog._create_session() 和 Config._from_environment_variables()，然後重現文件中記載的 REST 驗證環境變數範例。為支援的環境變數表示方式新增回歸測試覆蓋，並驗證 catalog 初始化能夠到達已設定的 auth manager，而不會將 auth 視為不可用的字串或點號鍵映射。

由索引模型根據 Issue 內容生成。