**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
if secrets_masker.should_hide_value_for_key(form.key.data):
            form.val.data = "*" * 8
    def prefill_show(self, item):
        if secrets_masker.should_hide_value_for_key(item.key):
            item.val = "*" * 8
    def _show(self, pk):
        pages = get_page_args()
        page_sizes = get_page_size_args()
        orders = get_order_args()
        item = self.datamodel.get(pk, self._base_filters)
        if not item:
            abort(404)
        self.prefill_show(item)
        widgets = self._get_show_widget(pk, item)
        self.update_redirect()
        return self._get_related_views_widgets(
            item, orders=orders, pages=pages, page_sizes=page_sizes, widgets=widgets
        )
    extra_args = {"can_create_variable": _can_create_variable}
    @action("muldelete", "Delete", "Are you sure you want to delete selected records?", single=False)
    def action_muldelete(self, items):