Prepare for changes to OnCall plugin initialization (#4715)

# What this PR does
Upcoming changes to OnCall plugin initialization will be expecting
`grafana_token` from all requests coming from the OnCall plugin. It is
not currently used it is being added so the plugin can tolerate the
update without errors.

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

grafana-plugin/src/plugin.json

@@ -143,7 +143,7 @@
       "headers": [
         {
           "name": "X-Instance-Context",
-          "content": "{ \"stack_id\": \"{{ printf \"%.0f\" .JsonData.stackId }}\", \"org_id\": \"{{ printf \"%.0f\" .JsonData.orgId }}\" }"
+          "content": "{ \"stack_id\": \"{{ printf \"%.0f\" .JsonData.stackId }}\", \"org_id\": \"{{ printf \"%.0f\" .JsonData.orgId }}\", \"grafana_token\": \"{{ .SecureJsonData.grafanaToken }}\" }"
         },
         {
           "name": "Authorization",