Create new experimental endpoints to update a given feature in an environment

[matthewelwell]

Related issues / PRs: #6221, #6029, #6279

We need to create 2 new endpoints which will allow us to have a single endpoint to update a feature in an environment, regardless of whether the environment has v2 feature versioning enabled or not.

The 2 new endpoints will look very similar with subtle differences to allow us to determine (via customer feedback) if we want both, or prefer one over the other. One endpoint will cater for updating both the environment default flag, as well as creating and updating segment overrides, all in a single request. The other will expect each atomic update to be done in an individual request. The expected payloads are given below.

Some other notes:

1. For these experimental endpoints, we are going to ignore multivariate updates

// Update all components of a feature in one request
{
  "feature": {
    // only one of these should be included in each request
    "name": "my_feature_name",
    "id": 1
  },
  "environment_default": {
    "enabled": true,
    "value": {
      "string_value": "",
      "type": "integer" // one of "integer", "string", "boolean", "float"
    }
  },
  "segment_overrides": [
    {
      "segment": {
        "id": 1,
        "priority": 1 // can be null / omitted, which will add it to the bottom of the list
      },
      "enabled": true,
      "value": {
        "string_value": "",
        "type": "integer"
        // one of "integer", "string", "boolean", "float"
      }
    }
  ]
}

// Update individual component of a feature in each request
{
  "feature": {
    // only one of these should be included in each request
    "name": "my_feature_name",
    "id": 1
  },
  "segment": {
    // since segment name is not unique (yet), we can't
    // offer name here like we do for feature
    "id": 1,
    "priority": 1 // can be null / omitted, which will add it to the bottom of the list
  },
  "enabled": true,
  "value": {
    "string_value": "",
    "type": "integer" // one of "integer", "string", "boolean", "float"
  },
}

[matthewelwell]

Next steps:

1. Update this issue to describe that we will create an experimental endpoint with the schema as proposed above
2. Create a new issue to describe that we will create another experimental endpoint with the previous schema (to make a single atomic update to a feature in an environment)
3. Implement those endpoints (probably excluding MV logic)
4. Gather all customers which are currently using an organisation API key to update flags, excluding those with MV flags
5. Email them with the information on why we're moving to versioning, and it's benefits, and ask if it fits their use cases

[matthewelwell]

Next steps:

1. Update this issue to describe that we will create an experimental endpoint with the schema as proposed above
2. Create a new issue to describe that we will create another experimental endpoint with the previous schema (to make a single atomic update to a feature in an environment)
3. Implement those endpoints (probably excluding MV logic)
4. Gather all customers which are currently using an organisation API key to update flags, excluding those with MV flags
5. Email them with the information on why we're moving to versioning, and it's benefits, and ask if it fits their use cases

See notion page here for further context.