Add mirror config support at nerdctl.toml

### What is the problem you're trying to solve

When I set the image mirror, I need to maintain many configs.

```bash
➜ nerdctl tree
.
├── buildkit
│   └── buildkitd.toml
├── containerd
│   └── certs.d
│   ├── docker.io
│   │   └── hosts.toml
│   ├── gcr.io
│   │   └── hosts.toml
│   └── quay.io
│   └── hosts.toml
└── nerdctl.toml
```

### Describe the solution you'd like

Add mirror config support at nerdctl.yaml like the `registries.yaml` of k3s
```yaml
mirrors:
"docker.io":
endpoint:
- "https://xxxx.mirror.aliyuncs.com"
- "https://mirrors.example.com"
"quay.io":
endpoint:
- "https://mirrors.example.com"
"gcr.io":
endpoint:
- "https://mirrors.example.com"

```

### Additional context

_No response_