schema/app: add annotation to expose sd_notify() support

The annotation "appc.io/executor/systemd/support-notify" exposes
if or not an application supports notifications using sd_notify().
This information may be used to signal the init process when a service is ready.

examples/image.json

@@ -117,6 +117,10 @@
         {
             "name": "homepage",
             "value": "https://example.com"
+        },
+        {
+            "name": "appc.io/executor/systemd/support-notify",
+            "value": "false"
         }
     ]
 }

schema/types/annotations_test.go

@@ -79,6 +79,12 @@ func TestAnnotationsAssertValid(t *testing.T) {
 			},
 			false,
 		},
+		{
+			[]Annotation{
+				makeAnno("appc.io/executor/systemd/support-notify", "false"),
+			},
+			false,
+		},
 		// empty is OK
 		{
 			[]Annotation{},

spec/aci.md

@@ -169,7 +169,7 @@ JSON Schema for the Image Manifest (app image manifest, ACI manifest), conformin
                 "count": 1000,
                 "protocol": "tcp"
             }
-        ]
+        ],
     },
     "dependencies": [
         {
@@ -211,6 +211,10 @@ JSON Schema for the Image Manifest (app image manifest, ACI manifest), conformin
         {
             "name": "homepage",
             "value": "https://example.com"
+        },
+        {
+            "name": "appc.io/executor/supportsSystemdNotify",
+            "value": false
         }
     ]
 }
@@ -250,6 +254,7 @@ JSON Schema for the Image Manifest (app image manifest, ACI manifest), conformin
     * **authors** contact details of the people or organization responsible for the image (freeform string)
     * **homepage** URL to find more information on the image (string, must be a URL with scheme HTTP or HTTPS)
     * **documentation** URL to get documentation on the image (string, must be a URL with scheme HTTP or HTTPS)
+    * **appc.io/executor/supportsSystemdNotify** (boolean, optional, defaults to "false" if unset) if set to true, the application MUST use the [sd_notify()](https://www.freedesktop.org/software/systemd/man/sd_notify.html) mechanism to signal when it is ready. An application with this parameter set to true MUST be able to detect if the executor had not set up the sd_notify mechanism and skip the notification without error ([sd_notify()](https://www.freedesktop.org/software/systemd/man/sd_notify.html) from libsystemd does that automatically).
 
 #### Dependency Matching