Add Abort Signal support for mutation and action

[eranknafo2001]

just like in trpc

[thomasballinger]

Which clients are you talking about? For ConvexHttpClient, sure.

For WebSocket clients, actions are send immediately on the WebSocket, so adding an abortSignal would require protocol work so send a cancellation message over the WebSocket and then try to cancel the action while it's running; this just doesn't seem that useful. But implementing this for mutations makes a little more sense: since mutations are queued, cancelling one is sometimes possible because the mutation hasn't even been sent yet.