feat: Stream Migrations/Merging

[fubuloubu]

Allow "migrating" one (or more) existing streams into one new stream, including all of the assets those old streams contain

The new stream will have new stream parameters including funding rate and product code which must be checked with the validator system again. Essentially, this is just a way to automate the action of closing N streams (owned by the same account) and creating a new stream with the given stream parameters, without having to transfer any unvested assets to do so.

Off-chain listeners will be able to see these events and consolidate those streams together in the off-chain payment service they are operating (for example, migrating a tier account + add-ons to a higher tier account that includes those add-ons). This should use a new StreamsMigrated event that doesn't trigger StreamCreated or StreamCancelled handling (which usually correlates with de-/provisioning resources in response to these stream events)

[linear]

SBK-473 feat: Stream Migrations/Merging