Feature request: --format flag for skaffold diagnose

`skaffold diagnose --yaml-only` is extremely useful for identifying issues with potentially complicated skaffold configurations. It would be fantastic if there were a way to get the errors in json format for easy consumption.

### Input Yaml

```yaml
apiVersion: skaffold/v2beta4
kind: Config
build:
tagPolicy:
sha256: {}
artifacts:
- image: java-hello-world
context: .
docker:
target: mm
deploy:
kubectl:
manifests:
- ./kubernetes-manifests/**.yaml
profiles:
- name: cloudbuild
some-invalid-yaml
build:
googleCloudBuild: {}
- name: dockerfile
build:
artifacts:
- image: java-hello-world
```

### Current output


### Proposed Output

`skaffold diagnose -f skaffold.yaml --yaml-only --format json` might output the following json

```json
{
"errors": [{
"line": 18,
"message": "could not find expected ':'"
}]
}
```