Add support for setting notification api key in headless mode

### Is this feature request related to a new or existing Amplify category?

notifications

### Is this related to another service?

_No response_

### Describe the feature you'd like to request

We would like to have the ability for setting the notification api key for example FCM via the continous integration pipeline. For example, we use gitflow to get new features into develop and production branches. Once we merge a pull request that is introducing notifications we are still forced to push this change manually because there is no way of setting the api key in headless mode as part of the integration pipeline.

### Describe the solution you'd like

We would like to be able to set the api key in headless mode.

### Describe alternatives you've considered

Creating a custom push script for ci/cd and setting the key via an environment variable set on the amplify console.
```
NOTIFICATIONSCONFIG="{\
\"Pinpoint\":{\
\"FCM\":{\
\"Enabled\":true,\
\"ApiKey\":\"${FCM_API_KEY}\"\
}\
}\
}"
```

### Additional context

_No response_

### Is this something that you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request
- [ ] ⚠️ This feature might incur a breaking change

No files, tests, or entry points are named. Trace the existing headless notification configuration and CI/CD handling to find where an FCM API key can be accepted; done means the key can be set through headless mode during an integration pipeline without manual notification setup.

Written by the indexing model from the issue text.