improve share validation

[plebhash]

@Shourya742 raised some concerns about the direction #1902 is going #1902 (comment)

a proposed alternative is to do all the persistence at the application layer, which is currently missing some information

this PR improves ShareValidationResult so that the application layer has access to the share hash

every other information relevant for some kind of persistence on the application layer can be retrieved from:

• the current channel state (e.g.: share_work is simply the current channel's Target, or user_identity can be retrieved via get_user_identity)
• the context where we're calling validate_share() (e.g.: channel_id and sequence_number is already available on the share message itself)
• the channel's internal ShareAccounting, which can be retrieved via get_share_accounting()

this also avoids some redundancies on the proposed solution of #1965

[average-gary]

wouldn't share_work be something related to the share and not the channel difficulty?

[plebhash]

a share should be rewarded because it meets the difficulty target established by the server, if the hash has more zeros than necessary, that doesn't make that share more valuable

that's the rationale we have always followed to update ShareAccounting::share_work_sum, which I got from an interaction with Braiins a while ago

btw, that's also the same rationale you're following on #1965, the only difference is that it's being returned inside ShareValidationResult::Valid, which as I pointed out is unnecessary, since that information is already available via get_target

https://github.com/stratum-mining/stratum/pull/1965/files#r2461063936

[plebhash]

please see stratum-mining/sv2-spec#126

for some reason the rendering of images on my fork is messed up, but I think this one summarizes things: