feat: add 'performance' label to all repos (#548)

As we focus more on performance issues like latency and memory usage
across our various services, it's going to be useful to have a
consistent, project-wide label to track these.

edx_repo_tools/repo_checks/labels.yaml

@@ -73,6 +73,10 @@
   color: "038E8E" # tealish
   description: "Relates to an Axim Funded Contribution project"
 
+- name: "performance"
+  color: "4a03fc" # blue-purple
+  description: "Relates to improving latency/throughput or reducing resource usage"
+
 
 ### LABELS INDICATING THE SCOPE OR FUNCTION OF THE ISSUE.
 ### AT MOST ONE OF THESE SHOULD BE USED AT A TIME.