[EuiResizableFlyout] Make size controllable

[CoenWarmer]

Is your feature request related to a problem? Please describe.
When using the resizable flyout, I need to be able to control its width. Use cases are:

• having a button which toggles the size of the flyout from full width to default width
• when having a toggleable sidebar inside the flyout, allowing the flyout to expand outwards when the sidebar is expanded, and become smaller again when the sidebar is collapsed.

Both use cases are (AFAIK) not possible to achieve without size being controlled.

Describe the solution you'd like
I'd love the size prop to be controllable.

Describe alternatives you've considered
I don't really think there is an alternative for this? -> add a key prop. This would remount the whole component, which considering the contents of the flyout in our use case is expensive.

Desired timeline
Having it somewhere a couple of days before 8.14 FF would be amazing. (Faster is welcome too of course :D)

Additional context
Slack conversation: https://elastic.slack.com/archives/C7QC1JV6F/p1708089823769239

[JasonStoltz]

Hey @CoenWarmer, just circling back to this one. Apologies for not getting to this yet -- we have this queued up as high-priority for the team, but we're past the requested 8.14 FF for this.

Just wanted to check in with you -- should we still consider this a high priority? From a product roadmap perspective, are there important user features that this is currently blocking? Just looking for some sort of indication around user value and/or immediate business needs for this to help gauge priority.

[CoenWarmer]

Hey @JasonStoltz, thanks for following up. No problem at all; we have since moved away from this pattern for now. Can be pushed to a lower priority as far as we are concerned. Thanks!

[JasonStoltz]

Thank you!

[PhilippeOberti]

@JasonStoltz I just chatted with @cee-chen and she pointed me to this issue. I have this exact case: in Security Solution, we have a flyout that can be expanded via a button inside the flyout. That way we can handle a right section which is a summary and a left section (expanded) to show a more detailed view.
For 8.15 we would love to be able to have this whole flyout resizable, but at the moment, we can programmatically resize the EuiResizableFlyout when clicking on our expand details button, unless we unmount and remount the whole flyout.

If this issue can be implemented for 8.15 that would be awesome, if not we'll wait a bit longer!
Thanks! 😄