**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
task_ids: list[str | tuple[str, int]] = []
        end_date = execution_date if not future else None
        start_date = execution_date if not past else None
        locked_dag_run_ids: list[int] = []
        if group_id is not None:
            task_group_dict = dag.task_group.get_task_group_dict()
            task_group = task_group_dict.get(group_id)
            if task_group is None:
                return redirect_or_json(
                    origin, msg=f"TaskGroup {group_id} could not be found", status="error", status_code=404
                )
            task_ids = task_ids_or_regex = [t.task_id for t in task_group.iter_tasks()]
            
            
            dag_runs_query = session.query(DagRun.id).filter(DagRun.dag_id == dag_id).with_for_update()
            if start_date is None and end_date is None:
                dag_runs_query = dag_runs_query.filter(DagRun.execution_date == start_date)
            else:
                if start_date is not None:
                    dag_runs_query = dag_runs_query.filter(DagRun.execution_date >= start_date)
                if end_date is not None:
                    dag_runs_query = dag_runs_query.filter(DagRun.execution_date <= end_date)
            locked_dag_run_ids = dag_runs_query.all()
        elif task_id:
            if map_indexes is None:
                task_ids = [task_id]
            else:
                task_ids = [(task_id, map_index) for map_index in map_indexes]
            task_ids_or_regex = [task_id]
        dag = dag.partial_subset(