**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
"action_varexport": "read",
    }
    base_permissions = [
        permissions.ACTION_CAN_CREATE,
        permissions.ACTION_CAN_READ,
        permissions.ACTION_CAN_EDIT,
        permissions.ACTION_CAN_DELETE,
        permissions.ACTION_CAN_ACCESS_MENU,
    ]
    list_columns = ["key", "val", "description", "is_encrypted"]
    add_columns = ["key", "val", "description"]
    edit_columns = ["key", "val", "description"]
    show_columns = ["key", "val", "description"]
    search_columns = ["key", "val"]
    base_order = ("key", "asc")
    def hidden_field_formatter(self):
        """Formats hidden fields"""
        key = self.get("key")
        val = self.get("val")
        if secrets_masker.should_hide_value_for_key(key):
            return Markup("*" * 8)
        if val:
            return val
        else:
            return Markup('<span class="label label-danger">Invalid</span>')
    formatters_columns = {
        "val": hidden_field_formatter,
    }
    validators_columns = {"key": [validators.DataRequired()]}
    def prefill_form(self, form, request_id):