**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
"""Runs Task Instance."""
        dag_id = request.form.get("dag_id")
        task_id = request.form.get("task_id")
        dag_run_id = request.form.get("dag_run_id")
        map_index = request.args.get("map_index", -1, type=int)
        origin = get_safe_url(request.form.get("origin"))
        dag: DAG = get_airflow_app().dag_bag.get_dag(dag_id)
        task = dag.get_task(task_id)
        ignore_all_deps = request.form.get("ignore_all_deps") == "true"
        ignore_task_deps = request.form.get("ignore_task_deps") == "true"
        ignore_ti_state = request.form.get("ignore_ti_state") == "true"
        executor = ExecutorLoader.get_default_executor()
        if not executor.supports_ad_hoc_ti_run:
            msg = f"{executor.__class__.__name__} does not support ad hoc task runs"
            return redirect_or_json(origin, msg, "error", 400)
        dag_run = dag.get_dagrun(run_id=dag_run_id)
        ti = dag_run.get_task_instance(task_id=task.task_id, map_index=map_index)
        if not ti:
            msg = "Could not queue task instance for execution, task instance is missing"
            return redirect_or_json(origin, msg, "error", 400)
        ti.refresh_from_task(task)
        
        dep_context = DepContext(
            deps=RUNNING_DEPS,