couchbase-cli user-manage should allow to view roles for single user

When reviewing security roles for a user, it would be useful to be able to perform a --my-roles type query but for another user.

We could get this via a --list and filter the result set externally, but this feature should ideally be baked in to the user-manage command. In an enterprise environment there could be a very large set of users and being unable to retrieve a single user is not really ideal.

Perhaps this could be implemented as a -get call along with -rbac-username to specify the user roles to query?

That is to say, usage could be something like:

couchbase-cli user-manage -c 127.0.0.1 -u Administrator -p password --get --rbac-username philstott

This would be ideal to have if we were programatically driving the new user configuration via couchbase-cli.