Support collapsing side navigation via ChromeStart

[TattdCodeMonkey]

Describe the feature:

There is currently a way to check the state of the side nav using getIsSideNavCollapsed$() from the ChromeStart object. But there is not a way to change the state from another plugin. It would be nice to have an accompanying function to either toggle the side nav, or better for my use case below a setIsSideNavCollapsed(value: boolean) to manually close the side nav from another plugin.

Describe a specific use case for the feature:

We are in the planning process for a new focused onboarding flow for Search users and would like to start with the side nav collapsed but still available if the user wants to open it then navigate to another page. We would use this feature to ensure the side nav is collapsed when loading the onboarding page.

[elasticmachine]

Pinging @elastic/appex-sharedux (Team:SharedUX)