fix(events): now EventBus.grantPutEventsTo correctly handles service principals (under feature flag) (#33729)

### Issue #22080

Closes #22080.

### Reason for this change

When trying to grant PutEvents permissions to an AWS Service Principal using `grantPutEventsTo`, the method performed a no-op without any warnings or errors. This prevented users from properly granting permissions to service principals, even though this is a valid use case that can be done through the AWS Console. The change implements the correct behavior by creating an EventBusPolicy when dealing with service principals.

### Description of changes

- Added special handling for service principals in `EventBus.grantPutEventsTo` method
- When granting permissions to a service principal, creates an EventBusPolicy instead of attempting to modify IAM policies
- Returns `iam.Grant.drop()` for service principals to indicate permissions are handled via EventBusPolicy
- Added test cases to verify both service principal and IAM principal scenarios

### Describe any new or updated permissions being added

The change introduces the creation of EventBusPolicy resources with `events:PutEvents` permission when granting access to service principals. This is not a new permission, but rather a different way of granting the same permission through resource-based policies instead of identity-based policies.


### Description of how you validated changes

Added new test cases that verify:
- EventBusPolicy is correctly created when granting permissions to service principals
- IAM policies are correctly created when granting permissions to IAM roles/users

### Checklist
- [X] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: scorbiere