[Dashboard] Allow App State Modifications Via URL

[ThomThomson]

When is Dashboard state read from URL

After #97941, in 7.14.0, dashboard state is only read and updated from the URL once on app load. The state is then removed from the URL and backed up in the session storage and redux state.

Some use cases, like #108433 (comment) require app state to be modified programatically at runtime by injecting variables into the URL. Before 7.14, these runtime state updates would be applied, after 7.14, modifying the URL without a hard refresh does not apply the state.

A possible solution
A subscription could be added which listens for changes from the app state in the URL, and overrides any state in the session storage & redux with the values injected via the URL. The _a param would subsequently be removed from the URL again, to keep the address bar clean. This would be possible in the new dashboard state architecture, and would allow programatic change of the entire dashboard state.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)