Add nested JSON fields for `=` and `:=` operators

[talis-fb]

The objective is to replicate this feature in HTTPie (https://httpie.io/docs/cli/nested-json).

  people[name]=Peter  \
  people[about][job]=dev \
  people[about][country]=Brazil \
  people[about][age]:=27 \
  people[skills]:=["Rust", "Python"]
---
{
    "people": {
        "name": "Peter",
        "about": {
            "job": "dev",
            "country": "Brazil",
            "age": 27
        },
        "skills": [
            "Rust",
            "Python"
        ]
    }
}

Additional Considerations:

• Ensure that the introduced operator is clearly documented and explained in the treq documentation.
• Conduct thorough testing to verify the accurate inference of JSON types and proper integration with the existing treq command.
• Consider providing informative error messages in case of invalid JSON structures.
• Note: It does NOT necessary add the building arrays through [] suffix like this FIRST EXAMPLE in httpie doc or this example...

  search[keywords][]=APIs \
  search[keywords][]=CLI
  ---------
  {
  "search": {
      "keywords": [
          "APIs",
          "CLI"
      ],
  }
  }

  Only focus in define nested values in this task.