S2S: Switch from Grant->Accept->Enable to Invite->Accept->Grant

Giving access now starts with an Invite activity, followed by Accept from the
Invite's recipient. Finally, the resource sends a Grant, which is the actual
OCap.

src/Web/ActivityPub.hs

@@ -71,6 +71,7 @@ module Web.ActivityPub
     , Create (..)
     , Follow (..)
     , Grant (..)
+    , Invite (..)
     , OfferObject (..)
     , Offer (..)
     , Push (..)
@@ -1507,13 +1508,32 @@ data Grant u = Grant
 parseGrant :: UriMode u => Object -> Parser (Grant u)
 parseGrant o =
     Grant
-        <$> o .: "object"
+        <$> o .:+ "object"
         <*> o .: "context"
         <*> o .: "target"
 
 encodeGrant :: UriMode u => Grant u -> Series
 encodeGrant (Grant obj context target)
-    =  "object"  .= obj
+    =  "object"  .=+ obj
+    <> "context" .= context
+    <> "target"  .= target
+
+data Invite u = Invite
+    { inviteInstrument :: Either Role (ObjURI u)
+    , inviteObject     :: ObjURI u
+    , inviteTarget     :: ObjURI u
+    }
+
+parseInvite :: UriMode u => Object -> Parser (Invite u)
+parseInvite o =
+    Invite
+        <$> o .:+ "instrument"
+        <*> o .: "object"
+        <*> o .: "target"
+
+encodeInvite :: UriMode u => Invite u -> Series
+encodeInvite (Invite obj context target)
+    =  "object"  .=+ obj
     <> "context" .= context
     <> "target"  .= target
 
@@ -1629,6 +1649,7 @@ data SpecificActivity u
     | CreateActivity  (Create u)
     | FollowActivity  (Follow u)
     | GrantActivity   (Grant u)
+    | InviteActivity  (Invite u)
     | OfferActivity   (Offer u)
     | PushActivity    (Push u)
     | RejectActivity  (Reject u)
@@ -1666,6 +1687,7 @@ instance ActivityPub Activity where
                         "Create"  -> CreateActivity  <$> parseCreate o a actor
                         "Follow"  -> FollowActivity  <$> parseFollow o
                         "Grant"   -> GrantActivity   <$> parseGrant o
+                        "Invite"  -> InviteActivity  <$> parseInvite o
                         "Offer"   -> OfferActivity   <$> parseOffer o a actor
                         "Push"    -> PushActivity    <$> parsePush a o
                         "Reject"  -> RejectActivity  <$> parseReject o
@@ -1691,6 +1713,7 @@ instance ActivityPub Activity where
         activityType (CreateActivity _)  = "Create"
         activityType (FollowActivity _)  = "Follow"
         activityType (GrantActivity _)   = "Grant"
+        activityType (InviteActivity _)  = "Invite"
         activityType (OfferActivity _)   = "Offer"
         activityType (PushActivity _)    = "Push"
         activityType (RejectActivity _)  = "Reject"
@@ -1702,6 +1725,7 @@ instance ActivityPub Activity where
         encodeSpecific _ _ (CreateActivity a)  = encodeCreate a
         encodeSpecific _ _ (FollowActivity a)  = encodeFollow a
         encodeSpecific _ _ (GrantActivity a)   = encodeGrant a
+        encodeSpecific _ _ (InviteActivity a)  = encodeInvite a
         encodeSpecific h u (OfferActivity a)   = encodeOffer h u a
         encodeSpecific h _ (PushActivity a)    = encodePush h a
         encodeSpecific _ _ (RejectActivity a)  = encodeReject a