Add NavigationAgent2D/3D set_navigation_map() function

[smix8]

Add NavigationAgent2D/3D set_navigation_map() and get_navigation_map() function to change maps with NavigationAgent Nodes as they do not react to changes on the abstract agent on the NavigationServer that are made with the API directly.

Fixes #62040

[fire-forge]

Since there is both a setter and a getter, this should probably be implemented as a property instead. It can be implemented as a non-editor property (PROPERTY_USAGE_NO_EDITOR) so that it's accessible from scripting but not in the inspector.

[smix8]

Wouldn't a property require another dedicated variable as the getter can return something different when no override is set?

[akien-mga]

In that case it's probably better as two separate methods as it's doing more than just setting a shared member variable indeed. The descriptions can't easily be unified into what happens when just reading a potential navigation_map or writing to it.

[akien-mga]

Thanks!

[akien-mga]

@smix8 Would be good if you can make a backport for this one to 3.5, it seems like there's still some minor difference in the NavigationAgent code between 3.x and master and I wouldn't want to mess it up (notably in set_agent_parent).