**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
DagRun.run_type == DagRunType.MANUAL,
                DagRun.conf.isnot(None),
            )
            .group_by(DagRun.conf)
            .order_by(func.max(DagRun.execution_date).desc())
            .limit(5)
            .all()
        )
        recent_confs = [getattr(run, "conf") for run in recent_runs]
        recent_confs = [conf[0] for conf in map(wwwutils.get_dag_run_conf, recent_confs) if conf[1]]
        if request.method == "GET":
            
            default_conf = ""
            doc_md = wwwutils.wrapped_markdown(getattr(dag, "doc_md", None))
            form = DateTimeForm(data={"execution_date": request_execution_date})
            if request_conf:
                default_conf = request_conf
            else:
                try:
                    default_conf = json.dumps(
                        {str(k): v.resolve(suppress_exception=True) for k, v in dag.params.items()},
                        indent=4,
                        ensure_ascii=False,
                    )
                except TypeError:
                    flash("Could not pre-populate conf field due to non-JSON-serializable data-types")
            return self.render_template(
                "airflow/trigger.html",
                dag_id=dag_id,
                origin=origin,