Feature request: provide mock capability for operator

### Feature Description

Hi team, I'm now heavily use opendal in my work, I'm wondering if it's possible to provide a mock `Operator`, so I could easily mock in my unit test for error testing?

I did some searches in the codebase, I find we do use mock-based unit tests for retry test:
https://github.com/apache/opendal/blob/faf12983a0f46a5c75641d1a0b0eaa5d0600b972/core/src/layers/retry.rs#L807-L1026

But that seems awkward and bespoke, I'm wondering if we could use [`mockall`](https://docs.rs/mockall/latest/mockall/) to decorate the trait, which unblocks full control for mock.

### Problem and Solution

```rust
use mockall::*;
use mockall::predicate::*;
#[automock]
trait Operator {
/// interface functions
}
```

### Additional Context

_No response_

### Are you willing to contribute to the development of this feature?

- [ ] Yes, I am willing to contribute to the development of this feature.

Start by locating the Operator trait and reading the mock-based retry tests in core/src/layers/retry.rs, especially the cited test range; compare that approach with mockall's documented #[automock] support. Done means the project has a supported way to create and configure a mock Operator for unit tests, including controlled error cases, with tests covering the capability.

Written by the indexing model from the issue text.