(appsync): Add ability to add additional authorization mode on existing appsyn api

### Describe the feature

Add support for additional authorization modes on existing app sync api (retrieved by calling `appsync.GraphqlApi.fromGraphqlApiAttributes`)

### Use Case

I have Amplify project with default Cognito user pool authorization mode. I want to add one more user pool as additional mode. Amplify does not support 2 user pools, that's why I use CDK to create one more user pool, and want to be able to add it as additional mode to my app sync api with CDK.

### Proposed Solution

Add new method (`addAdditionalAuthorizationModes`) on `IGraphqlApi` similar to `setupAdditionalAuthorizationModes` which will add additional authorizations modes to existing app sync api.

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.143.1

### Environment details (OS name and version, etc.)

-