chore: add missing codecov.yml file and comments

.github/actions/flutter_package/action.yaml

@@ -79,6 +79,11 @@ runs:
     - name: Upload Coverage
       uses: codecov/codecov-action@v3
       with:
+        # We use Codecov's carryforward flags to allow our PR testing to only
+        # run affected packages, but also allow our coverage information from
+        # past runs to "carry forward" on a "per flag" basis.  We tag coverage
+        # information with "flags" explaining what directory the coverage is from.
+        # https://docs.codecov.com/docs/carryforward-flags
         flags: ${{ steps.split.outputs.package_name }}
         token: ${{ inputs.codecov_token }}
 

.github/actions/rust_crate/action.yaml

@@ -36,5 +36,10 @@ runs:
     - name: Upload Coverage
       uses: codecov/codecov-action@v3
       with:
+        # We use Codecov's carryforward flags to allow our PR testing to only
+        # run affected packages, but also allow our coverage information from
+        # past runs to "carry forward" on a "per flag" basis.  We tag coverage
+        # information with "flags" explaining what directory the coverage is from.
+        # https://docs.codecov.com/docs/carryforward-flags
         flags: ${{ steps.split.outputs.package_name }}
         token: ${{ inputs.codecov_token }}

.github/codecov.yml

@@ -0,0 +1,13 @@
+# Not sure if .yaml suffix would work, currently using .yml to match examples:
+# https://docs.codecov.com/docs/codecov-yaml
+# Validate with:
+# curl -X POST --data-binary @codecov.yml https://codecov.io/validate
+# See https://docs.codecov.com/docs/flags
+flag_management:
+  default_rules: # the rules that will be followed for any flag added, generally
+    # We use Codecov's carryforward flags to allow our PR testing to only
+    # run affected packages, but also allow our coverage information from
+    # past runs to "carry forward" on a "per flag" basis.  We tag coverage
+    # information with "flags" explaining what directory the coverage is from.
+    # https://docs.codecov.com/docs/carryforward-flags
+    carryforward: true