st.server_state with endpoint access (like st.session_state, but for global values; different from pickled user sessions)

[sfc-gh-dmatthews]

Checklist

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Summary

Currently, we can use st.cache_data and st.cache_resource to "save" values that can be accessed between sessions. What if there was an API similar to st.session_state that was shared between sessions? I suggest st.server_state.

Additionally, a Streamlit app could include an endpoint for accessing and updating server state.

Why?

A common pattern is for a Streamlit app to reach out and collect data from a remote location, typically saving it with st.cache_data. If there was some server state combined with an endpoint, a remote source could be able to ping the app and initiate an update of data. This prevents needing to schedule an app to run to update data or making a random user wait if they are the first to connect beyond a cached values TTL.

This would also be useful for IoT use cases where a smart device can send an alert to the app.

Another feature request to send a global message to all sessions (#7312) could also be accommodated with this.

How?

Add a new API st.server_state which is global with all sessions having read/write access.
Add an (authenticated) enpoint for remote sources to connect to and update values in st.server_state.

Additional Context

This is related to requests for app state (#8609), but I'm suggesting something narrower.

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