Track start or end date for resources in a resource pool

Resources come and go in a resource pool and we need the ability to define start and end dates for people in a resource pool in order to better forecast our true capacity in Resource Planning. For example, a person might be changing roles after a certain date, or maybe is retiring. Or maybe we plan to hire X number of resources on a certain date and want to start planning for those future resources on our projects. When maintaining resources in a pool, users should be able to indicate a start or end date for any named resource. When viewing such resources in Resource Planning, the start and end date should drive when a resource is shown as available, not just the presence or lack of planned hours. At the project-level, those managing tasks should receive an error or warning if they try to assign resources not available for a given role in a certain timeframe. Time-off can be used as a workaround to some degree, but not in the case of a role change as of a certain date.