Add optional bounds type field to GetEntityBounds request

[ayushgnv]

Adds an optional type field to GetEntityBounds.srv so callers can specify their preferred bounding box representation (box, sphere, convex hull). If unset, it defaults to TYPE_UNSPECIFIED (255), preserving existing behavior where the simulator picks the type.

Also adds the TYPE_UNSPECIFIED constant to Bounds.msg as a clean sentinel value for use in requests, distinct from the existing response types.

Closes #26.

[peci1]

Is there some real use case for autoselection of the bounds type? I can't really imagine downstream code asking for "some" bounds.

But I agree with the addition of the type to the request

[ayushgnv]

Is there some real use case for autoselection of the bounds type? I can't really imagine downstream code asking for "some" bounds.

But I agree with the addition of the type to the request

@peci1
I was not sure if all simulators should be forced to support one of the types such as TYPE_BOX by default. A simulator could choose to have a different default from other simulators and so that's where this auto selection option might comes in handy.

If we agree as a standard to support a default type for all simulators, I'm happy with setting this option to that default type moving forward.