**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
map_indexes = args.getlist("map_index", type=int)
        upstream = to_boolean(args.get("upstream"))
        downstream = to_boolean(args.get("downstream"))
        future = to_boolean(args.get("future"))
        past = to_boolean(args.get("past"))
        origin = origin or url_for("Airflow.index")
        dag = get_airflow_app().dag_bag.get_dag(dag_id)
        if not dag:
            msg = f"DAG {dag_id} not found"
            return redirect_or_json(origin, msg, status="error", status_code=404)
        try:
            task = dag.get_task(task_id)
        except airflow.exceptions.TaskNotFound:
            msg = f"Task {task_id} not found"
            return redirect_or_json(origin, msg, status="error", status_code=404)
        task.dag = dag
        if state not in (
            "success",
            "failed",
        ):
            msg = f"Invalid state {state}, must be either 'success' or 'failed'"
            return redirect_or_json(origin, msg, status="error", status_code=400)
        latest_execution_date = dag.get_latest_execution_date()
        if not latest_execution_date:
            msg = f"Cannot mark tasks as {state}, seem that dag {dag_id} has never run"
            return redirect_or_json(origin, msg, status="error", status_code=400)
        if map_indexes is None:
            tasks: list[Operator] | list[tuple[Operator, int]] = [task]
        else:
            tasks = [(task, map_index) for map_index in map_indexes]