add always available streams (#5335)

When the publisher or source of a stream is offline, the server can be
configured to fill gaps in the stream with a video that is played on
repeat until a publisher comes back online. This allows readers to stay
connected regardless of the state of the stream. The offline video and
any future online stream are concatenated without decoding or
re-encoding packets, using the original codec.

api/openapi.yaml

@@ -377,6 +377,16 @@ components:
         useAbsoluteTimestamp:
           type: boolean
 
+        # Always available
+        alwaysAvailable:
+          type: boolean
+        alwaysAvailableFile:
+          type: string
+        alwaysAvailableTracks:
+          type: array
+          items:
+            $ref: '#/components/schemas/AlwaysAvailableTrack'
+
         # Record
         record:
           type: boolean
@@ -1088,6 +1098,20 @@ components:
           items:
             $ref: '#/components/schemas/SRTConn'
 
+    AlwaysAvailableTrack:
+      type: object
+      properties:
+        codec:
+          type: string
+        sampleRate:
+          type: integer
+          format: int64
+        channelCount:
+          type: integer
+          format: int64
+        muLaw:
+          type: boolean
+
     WebRTCICEServer:
       type: object
       properties: