Added graphql endpoint and tested with tags implementation.

notmuch/src/lib.rs

@@ -480,6 +480,11 @@ impl Notmuch {
         self.run_notmuch(std::iter::empty::<&str>())
     }
 
+    pub fn tags(&self) -> Result<Vec<String>, NotmuchError> {
+        let res = self.run_notmuch(["search", "--format=json", "--output=tags", "*"])?;
+        Ok(serde_json::from_slice(&res)?)
+    }
+
     pub fn search(
         &self,
         query: &str,