SendGrid: merge 'filters' in esp_extra

Previously, setting esp_extra['x-smtpapi']['filters']
would override the entire filters setting, potentially
undoing other Anymail options that use SendGrid
filters (like track_opens).

Now, 'filters' is special-cased, and merged with
any other Anymail filter options.

(We don't do a fully deep merge, because otherwise
there would be no way to use esp_extra to *clear*
Anymail settings.)

docs/esps/sendgrid.rst

@@ -117,7 +117,16 @@ Example:
         message.esp_extra = {
             'x-smtpapi': {
                 "asm_group": 1,  # Assign SendGrid unsubscribe group for this message
-                "asm_groups_to_display": [1, 2, 3]
+                "asm_groups_to_display": [1, 2, 3],
+                "filters": {
+                    "subscriptiontrack": {  # Insert SendGrid subscription management links
+                        "settings": {
+                            "text/html": "If you would like to unsubscribe <% click here %>.",
+                            "text/plain": "If you would like to unsubscribe click here: <% %>.",
+                            "enable": 1
+                        }
+                    }
+                }
             }
         }