[Detections] Endpoint Exceptions workflow should allow OS selection

[kevinlog]

Describe the feature:
We currently support both Mac and Windows Malware alerts. In the Exceptions workflow, there isn't an option to select a specific OS. As a result, when creating an Exception, it will be assigned to both Mac and Windows and the Endpoint will download an Exceptions artifact that contains Exceptions intended for either OS regardless of the OS the Endpoint is running on.

Exceptions modal, no OS selection:

After creating the Exception, you can see that it applies to both OSs:

The proposal here is to allow the user to select the OS in the Exceptions workflow so that they can differentiate as they see fit.

The pre-populated modal could pre-select an OS based on the OS that the Endpoint Alert originated from.

Describe a specific use case for the feature:

• As a user I want to maintain a set of Exceptions specific to Mac Endpoints that are not sent to Windows Endpoints.

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)