[Custom handlers] Add ability to specify level for each log message

Currently custom handler logs are an array of strings. Is it possible to allow setting the type (Error, Warning, etc) of the log message?

We could accept objects instead of strings. But this would be a breaking change.

Another possibility is to support prefixing the string with the level. Something like:

```json
"Logs": [
"[WARNING] this is a warning"
]
```

@pragnagopa @yojagad