**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
"""List loaded plugins."""
        plugins_manager.ensure_plugins_loaded()
        plugins_manager.integrate_executor_plugins()
        plugins_manager.initialize_extra_operators_links_plugins()
        plugins_manager.initialize_web_ui_plugins()
        plugins = []
        for plugin_no, plugin in enumerate(plugins_manager.plugins, 1):
            plugin_data = {
                'plugin_no': plugin_no,
                'plugin_name': plugin.name,
                'attrs': {},
            }
            for attr_name in self.plugins_attributes_to_dump:
                attr_value = getattr(plugin, attr_name)
                plugin_data['attrs'][attr_name] = attr_value
            plugins.append(plugin_data)
        title = "Airflow Plugins"
        doc_url = get_docs_url("plugins.html")
        return self.render_template(
            'airflow/plugin.html',
            plugins=plugins,
            title=title,
            doc_url=doc_url,
        )
class PoolModelView(AirflowModelView):