Create a solution for the placeholder on Event Stream

[james-prysm]

💎 Issue

the PR #13563 replaces the stream slots endpoint with an event stream, for the gRPC implementation it is actually an abstraction and still uses the underlying stream slots endpoint. In the future we will either need to get rid of gRPC and fully migrate to REST or create a temporary solution ( gRPC implementation of event stream) to migrate to. This next step will allow for us to take advantage of some topics such as slashing or reorgs on validator client.