[sumologicexporter]: add flatten_body to json logs

[sumo-drosiek]

Add merge_bodyoption which is going to flatten body if set to true. Example:

body:

{
  "a": "b",
  "c": "d"
}

attributes:

{
  "timestamp": 40
}

json_merge: false

{
  "log": {
    "a": "b",
    "c": "d"
  },
  "timestamp": 40
}

json_merge: true

{
  "a": "b",
  "c": "d",
  "timestamp": 40
}

[sumo-drosiek]

Uses open-telemetry/opentelemetry-collector-contrib#5676

[pmalek-sumo]

<hack></hack>