Cleaning up the rest of the API functions.

- Removed all unused imports
 - Fixed random compiler errors
 - Removed all of the 'sucess_response' helpers.
 - Enabled all of the API endpoints again, wrapping in 'into_boxfut'
 - Tidied up /metrics endpoint
 - Added a 'body_text' part to ResponseBuilder, mainly for the Prometheus /metrics endpoint
 - Cleaned up the unnecessary helpers::* imports, to be more explicit.

beacon_node/rest_api/src/response_builder.rs

@@ -61,4 +61,12 @@ impl ResponseBuilder {
             })?))
             .map_err(|e| ApiError::ServerError(format!("Failed to build response: {:?}", e)))
     }
+
+    pub fn body_text(self, text: String) -> ApiResult {
+        Response::builder()
+            .status(StatusCode::OK)
+            .header("content-type", "text/plain; charset=utf-8")
+            .body(Body::from(text))
+            .map_err(|e| ApiError::ServerError(format!("Failed to build response: {:?}", e)))
+    }
 }