[Beats] Enhancement to add support for string type of events.

We are planning to create an integration for Bitwarden and using different APIs provided by Bitwarden. We have encountered with an endpoint where we are getting a response in below format:

```
[
"3fa85f64-5717-4562-b3fc-2c963f66afa6"
]
```
When we configured same endpoint in Beats, we observed that in debug log we are getting below error message:

`events must be JSON objects, but got string: skipping`

We have also looked into [response.go](https://github.com/elastic/beats/blob/main/x-pack/filebeat/input/httpjson/response.go) file and observed that it is not able to convert input response to JSON object.