Add a CI step for checking app SDK codegen status (#111528)

* Add a CI step for checking app SDK codegen status

What

This commit adds a CI step for checking the status of code generated
with Grafana App SDK. The step fails if there is a git diff as a result
of the codegen step.

It also updates generated code to make sure we're starting from a
correct state.

Why

This ensures that when the schemas or the SDK version are updated, the
codegen mismatch is caught early at the PR stage.

Signed-off-by: Igor Suleymanov <igor.suleymanov@grafana.com>

* Format generated code

Signed-off-by: Igor Suleymanov <igor.suleymanov@grafana.com>

---------

Signed-off-by: Igor Suleymanov <igor.suleymanov@grafana.com>

.github/workflows/backend-code-checks.yml

@@ -58,6 +58,7 @@ jobs:
         run: |
           CODEGEN_VERIFY=1 make gen-cue
           CODEGEN_VERIFY=1 make gen-jsonnet
+          CODEGEN_VERIFY=1 make gen-apps
 
       - name: Validate go.mod
         run: go run scripts/modowners/modowners.go check go.mod