Introduce a Vault struct

[jorgeantonio21]

A Vault instance should encapsulate:

1. A user payment to access compute for a given task.
2. A node, or multiple nodes, selected to perform the compute task at hand, for the value paid.
3. Vaults should be transferrable, between users, and between nodes, if a node wants to unsubscribe to it, or a user declares a node to be unresponsive (Liveness failure).

[Cifko]

this will live in blockchain?
in the step 3, when is there a time for dispute? e.g. some nodes computes something, and the user claims it did not and removes it from the vault