Add realtime plugin

Add a plugin for View API realtime SSE channel (page streaming).

**NOTE: this commit removes `setPageAvailable`, `setPagesAvailableUpTo` and `setAllPagesAvailable` functionality from the viewer API in favor of the realtime plugin**. These methods were undocumented before, so this does not technically require a major version bump. We will need to make sure to communicate this change properly to anyone who might be affected.