**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
"""Retrieve logs including metadata."""
        dag_id = request.args.get("dag_id")
        task_id = request.args.get("task_id")
        execution_date = request.args.get("execution_date")
        map_index = request.args.get("map_index", -1, type=int)
        try_number = request.args.get("try_number", type=int)
        metadata = request.args.get("metadata", "{}")
        response_format = request.args.get("format", "json")
        
        try:
            metadata = json.loads(metadata)
            
            if not metadata:
                metadata = {}
        except json.decoder.JSONDecodeError:
            return {"error": "Invalid JSON metadata"}, 400
        
        try:
            execution_date = timezone.parse(execution_date)
        except ValueError:
            error_message = (
                f"Given execution date, {execution_date}, could not be identified as a date. "
                "Example date format: 2015-11-16T14:34:15+00:00"
            )
            return {"error": error_message}, 400
        task_log_reader = TaskLogReader()
        if not task_log_reader.supports_read:
            return {
                "message": "Task log handler does not support read logs.",