Data Transfer -- Unified Storage Validator

[hannahhoward]

Currently, we have two seperate validators for data transfer vouchers in storage market:

1. ClientRequestValidator
2. ProviderRequestValidator

The Client only accepts pulls while the Provider only accepts pushes. However, in GFC, we can only have one data transfer instance. So, we need to have a single RequestValidator that can be configured to operate like a Client or a Provider or both.

I would recommend something like:

type RequestValidator struct {
   acceptPushes bool
   acceptPulls bool
   deals *statestore.StateStore
}

Then in lotus we'll make two, one that accepts pushes & the other that accepts pulls
In GFC, we'll make only one that accepts both.