Add GridMap octant related queries and tests

[smix8]

Adds GridMap octant queries.

Requires #105329 to be merged first.

Adds octant query functions as a performance alternative to the existing cell functions.

E.g. when only a few cells for some specific octants are needed, e.g. inside some AABB bounds, and not all the cells of the entire GridMap which creates potentially a slow array with thousands of entries.

TypedArray[Vector3i] get_used_octants()
Returns the octant coordinates as Vector3i of all octants that have at least 1 cell in use.

TypedArray[Vector3i] get_used_octants_by_item( p_item: int)
Returns the used octant coordinates of all octants that have at least 1 cell with the specific cell item in use.

TypedArray[Vector3i] get_used_cells_in_octant(p_octant_coords: Vector3i)
Returns the used cell coordinates of all the cells inside the specific octant coordinate.

TypedArray[Vector3i] get_used_cells_in_octant_by_item(p_octant_coords: Vector3i, p_item: int)
Returns the used cell coordinates of all the cells inside the specific octant coordinate that have the specific cell item in use.

TypedArray[Vector3i] get_used_octants_in_bounds(p_bounds: AABB)
Returns the used octant coordinates of all octants that have at least 1 cell used with the specific local bounds.

Vector3i get_octant_coords_from_cell_coords(p_cell_coords: Vector3i)
Script helper to convert cell coordinate to octant coordinate for the octant related query functions.

[rburing]

Functionality looks good, and I would say the debug PR already justifies adding this API. I have just some comments on the docs.

[smix8]

I realized that I needed 2 native functions to get cells and octants inside AABBs that can skip Variant / TypedArray. Need them to speed up e.g. navmesh parsing or editor plugin that are really slow on larger GridMaps. So I added get_index_keys_in_bounds() and get_octant_keys_in_bounds().

Realizing further that so many new queries will turn into a nice bug surface at some point I also added some GridMap tests to cover the basics.