EventManager: Update EventManager to support trusting other accounts to place messages on it

[suhussai]

Describe the feature

To support account per tenant model, we need to be able to allow messages from eventbuses/resources in other accounts to place messages.

Use Case

This is useful in cases when users are trying to implement account per tenant.

Proposed Solution

No response

Other Information

No response

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

CDK version used

N/A

Environment details (OS name and version, etc.)

N/A