Add missing email type of connection (#18502)

The email configuration of Airflow became somwhat hybrid - part
of the configuration is in the config file and part (authentication)
in connection. However there is no dedicated connection type to
use for email, and user got confused - they did not realize they
could use any connection, and this was really not intuitive.

This PR adds email connection type as built-in and updates the
documentation of email configuration to use that connection
type for smtp/sendgrid (and use the aws one for aws email)

Fixes: #18495

airflow/www/views.py

@@ -3297,6 +3297,7 @@ def _get_connection_types() -> List[Tuple[str, str]]:
         _connection_types = [
             ('fs', 'File (path)'),
             ('mesos_framework-id', 'Mesos Framework ID'),
+            ('email', 'Email'),
         ]
         providers_manager = ProvidersManager()
         for connection_type, provider_info in providers_manager.hooks.items():

docs/apache-airflow/howto/email-config.rst

@@ -31,7 +31,7 @@ in the ``[email]`` section.
 
 To configure SMTP settings, checkout the :ref:`SMTP <config:smtp>` section in the standard configuration.
 If you do not want to store the SMTP credentials in the config or in the environment variables, you can create a
-connection called ``smtp_default``, or choose a custom connection name and set the ``email_conn_id`` with it's name in
+connection called ``smtp_default`` of ``Email`` type, or choose a custom connection name and set the ``email_conn_id`` with it's name in
 the configuration & store SMTP username-password in it. Other SMTP settings like host, port etc always gets picked up
 from the configuration only. The connection can be of any type (for example 'HTTP connection').
 
@@ -67,11 +67,17 @@ Airflow can be configured to send e-mail using `SendGrid <https://sendgrid.com/>
 
 Follow the steps below to enable it:
 
-1. Include ``sendgrid`` subpackage as part of your Airflow installation, e.g.,
+1. Include ``sendgrid`` provider as part of your Airflow installation, e.g.,
 
-  .. code-block:: ini
+  .. code-block:: bash
+
+     pip install 'apache-airflow[sendgrid]' --constraint ...
+
+or
+  .. code-block:: bash
+
+     pip install 'apache-airflow-providers-sendgrid' --constraint ...
 
-     pip install 'apache-airflow[sendgrid]'
 
 2. Update ``email_backend`` property in ``[email]`` section in ``airflow.cfg``, i.e.
 
@@ -82,7 +88,8 @@ Follow the steps below to enable it:
       email_conn_id = sendgrid_default
 
 3. Create a connection called ``sendgrid_default``, or choose a custom connection
-   name and set it in ``email_conn_id``.
+   name and set it in ``email_conn_id`` of  'Email' type. Only login and password
+   are used from the connection.
 
 .. _email-configuration-ses:
 
@@ -108,4 +115,4 @@ Follow the steps below to enable it:
       email_conn_id = aws_default
 
 3. Create a connection called ``aws_default``, or choose a custom connection
-   name and set it in ``email_conn_id``.
+   name and set it in ``email_conn_id``. The type of connection should be ``Amazon Web Services``.