Add NarrowPhase::set_query_dispatcher, skip serde for contact workspaces

[Jondolf]

Adds NarrowPhase::set_query_dispatcher. This is useful for snapshot restoration, because (de)serialization drops custom query dispatchers, and there's no other pre-existing way to set it without constructing a new NarrowPhase (as far as I know).

Also changes contact pairs to skip serialization for the contact workspace, which is transient and makes deserialization fail with "Cannot deserialize custom shape" for snapshots that contain custom shape contacts.

[sebcrozet]

Thank you!

[sebcrozet]

Actually, we shouldn’t skip the serialization of workspace as that could break determinism if the workspace contains warmstarting for some internal algorithm.

Instead, the deserialization panic should be addressed (I’m thinking we should have a mechanism that sets the workspace to a dummy value for unknown types. The specific contact algorithm would then automatically overwrite that dummy value with its own proper workspace type at its next execution.)