**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
children_states = {item["state"] for item in child_instances}
            group_state = next((state for state in wwwutils.priority if state in children_states), None)
            group_start_date = min(filter(None, children_start_dates), default=None)
            group_end_date = max(filter(None, children_end_dates), default=None)
            return {
                "task_id": task_group.group_id,
                "run_id": dag_run.run_id,
                "state": group_state,
                "start_date": group_start_date,
                "end_date": group_end_date,
            }
        def get_mapped_group_summaries():
            mapped_ti_query = (
                session.query(
                    TaskInstance.task_id, TaskInstance.state, TaskInstance.run_id, TaskInstance.map_index
                )
                .filter(
                    TaskInstance.dag_id == dag.dag_id,
                    TaskInstance.task_id.in_(child["id"] for child in children),
                    TaskInstance.run_id.in_(r.run_id for r in dag_runs),
                )
                .order_by(TaskInstance.task_id, TaskInstance.run_id)
            )
            
            mapped_tis: Mapping[str, Mapping[int, list[TaskInstance]]] = collections.defaultdict(
                lambda: collections.defaultdict(list),
            )
            for ti in mapped_ti_query:
                mapped_tis[ti.run_id][ti.map_index].append(ti)
            def get_mapped_group_summary(run_id: str, mapped_instances: Mapping[int, list[TaskInstance]]):