**Description**

Replace the Configuration JSON text-area with a form that has individual input fields for each key & value. This will remove the need for the user to provide a json object. See hashicorp vault's key-value secrets UI for an example of what this might look like:

![Screenshot 2020-09-21 at 12 06 03](https://user-images.githubusercontent.com/17148324/93759844-df04f680-fc02-11ea-8613-f3bcc0b22d09.png)
 
- If present these keys and values should be serialised as json client-side and posted to the trigger-dag API endpoint as the `conf` request-parameter.
- By default no conf request-parameter is sent. Perhaps a checkbox should be used to determine whether conf is sent at all?
- The user may want to add new keys, or remove keys & values that they have previously added.
- [PR-10839](https://github.com/apache/airflow/pull/10839) is pre-populating the form with the DAG's default params. The new form should be pre-populated with the keys & values from `DAG.params`


**Use case / motivation**

Currently when triggering a manual DAG run with user-provided runtime parameters, the user has to provide a correctly formatted JSON object. Filling in a form with keys & values will be a significantly better user experience, and will be less error prone & laborious than handling json in a free-text field.

This builds on the great work already done on enhancing the Trigger-DAG functionality

**Related Issues**

Originating PR: https://github.com/apache/airflow/pull/10839
def rendered_k8s(self, *, session: Session = NEW_SESSION):
        """Get rendered k8s yaml."""
        if not settings.IS_K8S_OR_K8SCELERY_EXECUTOR:
            abort(404)
        dag_id = request.args.get("dag_id")
        task_id = request.args.get("task_id")
        if task_id is None:
            logging.warning("Task id not passed in the request")
            abort(400)
        execution_date = request.args.get("execution_date")
        dttm = _safe_parse_datetime(execution_date)
        form = DateTimeForm(data={"execution_date": dttm})
        root = request.args.get("root", "")
        map_index = request.args.get("map_index", -1, type=int)
        logging.info("Retrieving rendered templates.")
        dag: DAG = get_airflow_app().dag_bag.get_dag(dag_id)
        task = dag.get_task(task_id)
        dag_run = dag.get_dagrun(execution_date=dttm, session=session)
        ti = dag_run.get_task_instance(task_id=task.task_id, map_index=map_index, session=session)
        pod_spec = None
        try:
            pod_spec = ti.get_rendered_k8s_spec(session=session)
        except AirflowException as e:
            msg = "Error rendering Kubernetes POD Spec: " + escape(e)
            if e.__cause__:
                msg += Markup("<br><br>OriginalError: ") + escape(e.__cause__)
            flash(msg, "error")
        except Exception as e:
            flash("Error rendering Kubernetes Pod Spec: " + str(e), "error")
        title = "Rendered K8s Pod Spec"