Add optional merge field to FilesystemSave metadata

Signed-off-by: mulhern <amulhern@redhat.com>

src/engine/strat_engine/serde_structs.rs

@@ -199,6 +199,13 @@ pub struct FilesystemSave {
     pub fs_size_limit: Option<Sectors>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub origin: Option<FilesystemUuid>,
+    // if self.origin is None, then self.merge must be None
+    // if self.origin is Some,
+    //   then self.merge is None is equivalent to self.merge is Some(false)
+    //   this definition is fully backward compatible.
+    // TODO: This data type should no longer be optional in Stratis 4.0
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub merge: Option<bool>,
 }
 
 #[cfg(test)]

src/engine/strat_engine/thinpool/filesystem.rs

@@ -390,6 +390,7 @@ impl StratFilesystem {
             created: self.created.timestamp() as u64,
             fs_size_limit: self.size_limit,
             origin: self.origin,
+            merge: None,
         }
     }