Investigate the implementation of subscribing to certain Witness data

[ra0x3]

• Witness data are arbitrarily-sized byte arrays included in transactions that can include different types of data
• Since Witness data is just bytes, we should investigate whether or not these bytes can be used for subscriptions
• Currently we only have one type of subscription - contract subscriptions
• However if we implement Witness data subscriptions (which would work much the same as contract subscriptions), users would be able to index certain events based on signaled witness data