Add ability to pin new K7 side nav

[ryankeairns]

Describe the feature:
As a follow up to #28940 , we would like to add the ability to pin the side nav to its minimized/collapsed state.

Describe a specific use case for the feature:
This pinning would allow users to avoid the hover-to-expand interaction when they are familiar with the app icons and don't want to see it fly open. For links in the side nav that open the secondary menu (e.g. Recently viewed), clicking would still expand both the menu and flyout sections, overlaying the body content just as it does currently (in the aforementioned PR).

[elasticmachine]

Pinging @elastic/kibana-platform

[elasticmachine]

Pinging @elastic/kibana-design

[alexfrancoeur]

I'm a bit out of the loop on this one but I think it's important to get right for 7.0. @epixa @jinmu03 do we have anyone focused on this on the platform team?

[epixa]

We do not, as far as I'm aware. We're way past feature freeze, I don't think we should be putting this change into 7.0.

[snide]

We have a very small EUI fix that we'll be adding for this in the next week. We've had enough people complain that I think it's worth addressing. It will have zero engineering impact, not require local storage and should be non destructive. @ryankeairns is already talking with @alexfrancoeur about it.

I'll remove the platform label for now.

[alexfrancoeur]

@snide nice! I agree this functionality is definitely something we need in for 7.0

[cchaos]

A locking mechanism was added to the nav drawer's collapse button