`@opExample` custom metadata

### Clear and concise description of the problem

I am looking to add some custom metadata to an `@opExample`, but I can't seem to find a good way to do it. For example, I am trying to do something like this:
```tsp
@opExample(#{
returnType: #{
statusCode: 200,
body: #{
...
}
}
}, #{
`x-parameter-values`: #{
id: "abc123",
fields: "summary",
},
})
```
which would result in an openapi3 emitter output of
```yaml
examples:
example0:
x-parameter-values:
id: abc123
fields: summary
value:
...
```

### Checklist

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- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.