Support `flow.generateReport('json')` in user flows

### FAQ

- [X] Yes, my issue is not about [variability](https://github.com/GoogleChrome/lighthouse/blob/main/docs/variability.md) or [throttling](https://github.com/GoogleChrome/lighthouse/blob/main/docs/throttling.md).
- [X] Yes, my issue is not about a specific accessibility audit (file with [axe-core](https://github.com/dequelabs/axe-core) instead).

### URL

https://www.mi-perftest1.com/default.mi?phoenix=true

### What happened?

I am trying to generate Report in JSON format but report is being generated with empty {} brackets. below is the line of code.
And also could you suggest to generate report in csv format.

const jsonReport = flow.generateReport('json');
writeFileSync('myfile1.json', JSON.stringify(jsonReport));

### What did you expect?

Lighthouse report to be generated in JSON / CSV format

### What have you tried?

_No response_

### How were you running Lighthouse?

node

### Lighthouse Version

10.0.0

### Chrome Version

110.0.5481.77

### Node Version

v18.14.0

### OS

Windows

### Relevant log output

_No response_