**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
"""View to show Airflow Plugins"""
    default_view = 'list'
    class_permission_name = permissions.RESOURCE_PLUGIN
    method_permission_name = {
        'list': 'read',
    }
    base_permissions = [
        permissions.ACTION_CAN_READ,
        permissions.ACTION_CAN_ACCESS_MENU,
    ]
    plugins_attributes_to_dump = [
        "hooks",
        "executors",
        "macros",
        "admin_views",
        "flask_blueprints",
        "menu_links",
        "appbuilder_views",
        "appbuilder_menu_items",
        "global_operator_extra_links",
        "operator_extra_links",
        "source",
    ]
    @expose('/plugin')
    @auth.has_access(
        [
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_PLUGIN),
        ]
    )
    def list(self):