Integrate CodeCov as change acceptance stage

As a rough measure to keep quality up we want to integrate our code
coverage tooling to the point where it is required for changes to be
merged.

Example taken from https://github.com/cosmos/voyager/blob/develop/codecov.yaml

codecov.yml

@@ -0,0 +1,16 @@
+coverage:
+  precision: 2
+  round: down
+  range: "70...100"
+
+  status:
+    project: on
+    patch: on
+    changes: off
+
+comment:
+  layout: "diff, files"
+  behavior: default
+  require_changes: no
+  require_base: no
+  require_head: yes