[FEATURE] Add UI support to add existing roles while creating a user

[derek-ho]

Is your feature request related to a problem?
Currently, mapping a user to a role via the UI involves two steps:

1. Create the user
2. Map the user to the role

https://opensearch.org/docs/latest/security/access-control/api/#create-user - there is existing functionality that allows you to pass in existing roles when creating a user - this should be available on the UI as well

What solution would you like?
A clear and concise description of what you want to happen.

What alternatives have you considered?
A clear and concise description of any alternative solutions or features you've considered.

Do you have any additional context?
Add any other context or screenshots about the feature request here.

[cwperks]

[Triage] Thank you for filing the issue @derek-ho. If you have any suggestions for UX around the issue please add to the description to make it clear what acceptance for this issue would look like.