Allow merging state in multi-app environments

[eliias]

Problem

For certain setups, we must support "merging" state that is extracted from several render calls. For example, multiple Vue apps using Apollo or a state management library will produce a normalized hash map of entities. We should provide a simple (optimistic) merge algorithm for multi-apps, where we assume no conflicts, but merge into a single client.

[eliias]

This requires shared state between one or more views in Rails. We need to aggregate all states and merge them optimistically (last one wins). We can extract the state help (isorun_state) into the parent view/layout to avoid injecting more than once.

An app that is rendered with a given state_id, will not render the state immediately, but persist the state returned by the SSR function in a shared store, and isorun_state will simply fetch and merge all the states present in this variable.

# app_frame.html.erb
<%= isorun_app("sidebar", state_id: :apollo) %>
<%= isorun_app("main", state_id: :apollo) %>

# layout.html.erb
<%= isorun_state("apollo") %>