**Apache Airflow version**:
2.1.0

**What happened**:
There are several hooks that still use optional parameters from the classic `Extra` field.  However, when creating the connection the `Extra` field is overwritten with values from the custom fields that are included in the form.  Because the `Extra` field is overwritten, these optional parameters cannot be used by the hook.

For example, in the `AzureDataFactoryHook`, If `resource_group_name` or `factory_name` are not provided when initializing the hook, it defaults to the value specified in the connection extras. Using the Azure Data Factory connection form, here is the initial connection submission:
![image](https://user-images.githubusercontent.com/48934154/127029091-a8b1c979-f238-4bac-9d3a-f94b6cd92a2b.png)

After saving the connection, the `Extra` field is overwritten with the custom fields that use "extras" under the hood:
![image](https://user-images.githubusercontent.com/48934154/127029242-79c990f0-b9cf-43bb-9a6b-28b4e9c6d4e1.png)


**What you expected to happen**:

Wavering slightly but I would have initially expected that the `Extra` field wasn't overwritten but updated with both the custom field "extras" plus the originally configured values in the `Extra` field.  However, a better UX would be that the values used in the `Extra` field should be separate custom fields for these hooks and the `Extra` field is hidden.  Perhaps it's even both?

**How to reproduce it**:
Install either the Microsoft Azure or Snowflake providers, attempt to create a connection for either the Snowflake, Azure Data Factory, Azure Container Volume, or Azure types with the `Extra` field populated prior to saving the form.


**Anything else we need to know**:
Happy to submit PRs to fix this issue. 🚀 


airflow/www/views.py
(permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_CODE),
        ]
    )
    @provide_session
    def code(self, session=None):
        """Dag Code."""
        all_errors = ""
        dag_orm = None
        dag_id = None
        try:
            dag_id = request.args.get('dag_id')
            dag_orm = DagModel.get_dagmodel(dag_id, session=session)
            code = DagCode.get_code_by_fileloc(dag_orm.fileloc)
            html_code = Markup(highlight(code, lexers.PythonLexer(), HtmlFormatter(linenos=True)))
        except Exception as e:
            all_errors += (
                "Exception encountered during "
                + f"dag_id retrieval/dag retrieval fallback/code highlighting:\n\n{e}\n"
            )
            html_code = Markup('<p>Failed to load file.</p><p>Details: {}</p>').format(escape(all_errors))
        return self.render_template(
            'airflow/dag_code.html',
            html_code=html_code,
            dag=dag_orm,
            title=dag_id,
            root=request.args.get('root'),
            wrapped=conf.getboolean('webserver', 'default_wrap'),
        )
    @expose('/dag_details')
    @auth.has_access(