Add renewedTo field to RPCLastestRevision response

[ChrisSchinnerl]

In v2 a renter has no way of knowing whether a contract has been renewed when fetching a revision. So if a renter for some reason misses the fact that the contract was renewed (e.g. due to a crash mid-renewal), it would be nice of the host to tell the renter about it via a renewedTo field. Previously we used the max revision number for that but we don't have that anymore.

[ChrisSchinnerl]

@n8maninger I created this issue since it is something we discussed and after thinking about it I think it would be useful to have. Makes it easier to avoid failed interactions with hosts and also some of the contract maintenance checks.

[n8maninger]

Thinking about this more: instead of a RenewedTo field, maybe a "Resolution" field that includes the type? If resp.Resolution == "renewal", you can calculate the renewed contract ID using the existing contract ID: renewalID := fcid.V2RenewalID()

[ChrisSchinnerl]

Sounds good to me. I don't feel strongly about either approach. The resolution might be a bit nicer since we only need to send another byte.

[lukechampine]

Perhaps make it RPCContractStatus and return something like ForEachV2FileContractElement?

struct {
    Contract   types.V2FileContractElement // latest on-chain revision
    Revision   *types.V2FileContractRevision // latest off-chain revision, if it exists
    Resolution types.V2FileContractResolutionType // non-nil if contract has been resolved
}