Avoid generic parameter in CosmosMsg<C>

[chipshort]

We still need to support the custom message feature, but having this as a generic is quite annoying for users because it has to be carried through the whole code.

In our meeting, switching to dynamic dispatch with erased_serde / typetag was proposed.

We should make sure that downstream users are not affected by the fact that this makes CosmosMsg no longer Send + Sync and that the custom message type is not necessarily Sized anymore.
This could probably be looked at for something like MultiTest.