Caching CompatibleSilo information

[rkargMsft]

As noted here #9271 (comment)

PlacementService currently calls GetCompatibleSilos for every activation. Some of the information appears to be cached already, but filters will be run every placement.

Determine if this call can be further cached at any level.

Note that if user supplied Placement Filters exist then it is possible that the call may not be able to be cached. Also, when the cluster membership changes any prior cache could be invalid.