ci: add configuration fileds to codecov.yaml (tikv#6666)

ref PingCAP-QE/ci#2171, close tikv#6667

ci: add configuration fileds to codecov.yaml

- `ignore`: ignore integration test cases or tools paths.
- `flags`: to split static for unit and integration testing.
- `comment`: add `flags` section in comments.

Signed-off-by: wuhuizuo <[email protected]>

codecov.yml

@@ -10,3 +10,20 @@ coverage:
         # basic
         target: auto
         threshold: 3%
+
+comment:
+  layout: "header, diff, flags"
+  behavior: default
+  require_changes: false
+
+flag_management:
+  default_rules: # the rules that will be followed for any flag added, generally
+    carryforward: true
+    statuses:
+      - type: project
+        target: 85%
+      - type: patch
+        target: 85%
+
+ignore:
+  - tests/** # integration test cases or tools.