**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
"""Queue DagRun so tasks that haven't run yet can be started."""
        dag_id = request.form.get("dag_id")
        dag_run_id = request.form.get("dag_run_id")
        confirmed = request.form.get("confirmed") == "true"
        return self._mark_dagrun_state_as_queued(dag_id, dag_run_id, confirmed)
    @expose("/dagrun_details")
    def dagrun_details(self):
        """Redirect to the GRID DAGRun page. This is avoids breaking links."""
        dag_id = request.args.get("dag_id")
        run_id = request.args.get("run_id")
        return redirect(url_for("Airflow.grid", dag_id=dag_id, dag_run_id=run_id))
    def _mark_task_instance_state(
        self,
        *,
        dag_id: str,
        run_id: str,
        task_id: str,
        map_indexes: list[int] | None,
        origin: str,
        upstream: bool,
        downstream: bool,
        future: bool,
        past: bool,
        state: TaskInstanceState,
    ):
        dag: DAG = get_airflow_app().dag_bag.get_dag(dag_id)
        if not run_id:
            flash(f"Cannot mark tasks as {state}, seem that DAG {dag_id} has never run", "error")
            return redirect(origin)
        altered = dag.set_task_instance_state(