FIP-0083: add "client" to verifier-balance event

[rvagg]

Further to https://github.com/filecoin-project/FIPs/pull/964, this is a proposed change to FIP-0083 that we'd like to slip in to the current network upgrade in order to improve observability for certain observers of the network based on feedback we have received while trying to assist with nv22 integration.

The change here is simply to add a "client" field to the existing verifier-balance event emitted from the verified registry actor. This field will be null for cases where a verifier's allowance does not change as a result of allocation to a verified client, but will be the actor ID of the client where a client is involved.

Ultimately, in discussion, some of us have come to the conclusion that this event was not properly considered when originally implemented and does not allow for straightforward tracking of verifier allowances, and in its current form doesn't help connecting relationships and flows between verifiers and clients. Rather than attempting to augment or fix this event, we will likely propose a new FIP after this network upgrade to replace this event entirely with a new suite of events focused on datacap allowance and allocation flows to and from verifiers. e.g.

• add-allowance - emitted when a verifier's allowance is set using AddVerifier, possibly including a delta and final balance since this method can be used to initialise, add or even decrease an allowance.
• allocate-datacap - emitted when a verifier allocates datacap to a verified client, including the client (the "client" in this proposal), the amount allocated, and the final balance of the verifier's allowance.
• remove-verifier - emitted when a verifier is removed from the verified registry, possibly including the final balance of the verifier's allowance.

Additional work could be done in the datacap actor to add observability to flows on the client side.

In the meantime, we believe that adding the field in this PR to the existing verifier-balance field will provide a lightweight but useful stopgap for the nv22 integration and other observers of the network.

Discussion thread specifically for this change is: https://github.com/filecoin-project/FIPs/discussions/754#discussioncomment-8833387

[jennijuju]

Editor Review: Thank you for the detailed PR description. This is an enhancement for the proposed FIP to be useful for tooling builders and is a well contained change. Approved.

[aarshkshah1992]

lgtm