feat: allow reassignment of MapController to FlutterMap multiple times

[josxha]

Description

Allows the MapController to get assigned multiple times to a view (only one a every time).
Can't tell much about potential side effects. Did some quick tests and everything worked alright so far but additional testing would be good.

Issue

• [BUG] External MapController lifecycle not synced with widget when destroyed and rebuilt #1892

[pagetronic]

Perfect for me.

[JaffaKetchup]

LGTM I think. Thanks :)

Tbh I'm not 100% confident that all of the users experiencing this issue are using the MapController as per the documentation in the first place, especially when external state is involved, users seem to commonly get the setup wrong by initialising/constructing/defining the MapController in the state initially, then passing that to the map, rather than the reverse: https://docs.fleaflet.dev/usage/programmatic-interaction/external-custom-controllers#usage-within-a-state-system-model. Therefore, I'm not planning to change the documentation for this: I'm not sure how this will affect the need for those instructions, but they should ensure more stability than other options for the future.

[josxha]

Tbh I'm not 100% confident that all of the users experiencing this issue are using the MapController as per the documentation in the first place, especially when external state is involved

Yes, I agree. All the reported issues that I've seen on this were caused by incorrect usage of the MapController. But anyways, now we have unofficial support for those implementations. 👍

Thanks for the code review (: