Dispatch (capability, operation, params) to the currently-installed
HostCallBridge, if any. Some(Ok(_)) means the bridge handled the
call; Some(Err(_)) means it tried but raised; None means there is
no bridge or the bridge declined this op (returned Ok(None)).
Install a bridge for the current thread. The bridge is consulted on
every host_callafter mock matching but before the built-in
match arms, so embedders can override anything they like (and equally
punt on anything they don’t, by returning Ok(None)).