Support APK signing scheme v4

### Describe what you wanted to do

- please support [APK signing scheme v4](https://source.android.com/security/apksigning/v4)

### Describe what you expected

- it has `is_signed_v4()` method in `apk.py`

### Describe what actually happened

- according to [apk.py#L1569](https://github.com/androguard/androguard/blob/8d091cbb309c0c50bf239f805cc1e0931b8dcddc/androguard/core/bytecodes/apk.py#L1569), the code only checks v1 to v3 scheme, not v4

### System Information

- Androguard Version: 3.4.0a1
- Python Version: 3.8
- Operating System: Ubuntu 20.04LTS

### Further Log Files and Output

- I use androguard via [MobSF](https://mobsf.github.io/docs/#/) with [their official Docker image](https://hub.docker.com/r/opensecurity/mobile-security-framework-mobsf/), and it lacks v4 signature check feature
- I asked it in the MobSF slack and Ajin kindly suggest to report it here