**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
"""Home view."""
        unit_test_mode: bool = conf.getboolean('core', 'UNIT_TEST_MODE')
        if not unit_test_mode and "sqlite" in conf.get("core", "sql_alchemy_conn"):
            db_doc_page = get_docs_url("howto/set-up-database.html")
            flash(
                Markup(
                    "Usage of <b>SQLite</b> detected. It should only be used for dev/testing. "
                    "Do not use <b>SQLite</b> as metadata DB in production. "
                    "We recommend using Postgres or MySQL. "
                    f"<a href='{db_doc_page}'><b>Click here</b></a> for more information."
                ),
                category="warning",
            )
        if not unit_test_mode and conf.get("core", "executor") == "SequentialExecutor":
            exec_doc_page = get_docs_url("executor/index.html")
            flash(