🚀[FEATURE]: Add support for dynamic ngxsForm path

[Andreas-Hjortland]

Relevant Package

This feature request is for @ngxs/form-plugin

Description

When I change the [ngxsForm] property on the directive dynamically, it does not change the listeners on the store to reflect my changes.

Describe the problem you are trying to solve

I have an app with multiple tabs with forms which I would like to allow the user to jump between without saving (for instance to cross reference another entry before saving). When the user jumps to another tab the form is not updated because the form plugin is still bound to the old form since angular reuses my components and the `ngxsForm` directive only use the initial value of the path.

I have created a small project to illustrate the problem here. If you navigate to Superman and then to Batman, the form is still showing the values from the superman route. I have added the devtool plugin so if you open the app in a new window, you can see how the form plugin is a bit confused with where in the state it should read and write the form values after the initial load.

Describe the solution you'd like

I would like the ngxs form directive to dynamically update it's listeners whenever we change the path property on the `ngxsForm` attribute.

What I believe we have to do is to replace the ngOnInit method with an ngOnChanges method which is filtered on the path parameter and will unsubscribe the existing subscriptions whilst resubscribing with the new path whenever we change it.

Describe alternatives you've considered

I havent tried this, but I think we could replace the RouteReuseStrategy of our app to never reuse the components when the users are navigating between the same components with different parameters, but I believe that to be a sub-optimal solution since we are then opting out of some otherwise great performance and accessibility features like keeping the scroll position and other browser state when a user is navigating between entities of the same type.

---

If you would like, I can try to create a pull request with my proposed solution, and if you have some feedback on my proposal I can try to incorporate that as well =)

[markwhitfeld]

Thank you for the suggestion. I think that there is some merit in this request, but we need to make sure that it has general, and reasonable application, as well as no negative side effects for the regular use cases.
Could you think through what the drawbacks of a dynamic ngxsForm path could be?
If you would like to attempt a PR, then that would be great. Please bear in mind that any PRs would need a full suite of tests to demonstrate the feature. Given this fact, it would be good to clear up any finer points of discussion before proceeding in case there are any fundamentally blocking issues.

[markwhitfeld]

Discussion #1912 also seems to allude to this need.
So, if the dynamic value was set to null, then would the binding be disabled? Worth having a think about what this would mean as well.