**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
session.query(TaskInstance.state, sqla.func.count(TaskInstance.dag_id))
            .filter(TaskInstance.dag_id == dag_id)
            .group_by(TaskInstance.state)
            .all()
        )
        active_runs = models.DagRun.find(dag_id=dag_id, state=State.RUNNING, external_trigger=False)
        tags = session.query(models.DagTag).filter(models.DagTag.dag_id == dag_id).all()
        
        owner_links = session.query(DagOwnerAttributes).filter_by(dag_id=dag_id).all()
        attrs_to_avoid = [
            "schedule_datasets",
            "schedule_dataset_references",
            "task_outlet_dataset_references",
            "NUM_DAGS_PER_DAGRUN_QUERY",
            "serialized_dag",
            "tags",
            "default_view",
            "relative_fileloc",
            "dag_id",
            "description",
            "max_active_runs",
            "max_active_tasks",
            "schedule_interval",
            "owners",
            "dag_owner_links",
            "is_paused",
        ]
        attrs_to_avoid.extend(wwwutils.get_attr_renderer().keys())
        dag_model_attrs: list[tuple[str, Any]] = [
            (attr_name, attr)