Extension: PPLNS with Job Declaration

[plebhash]

DEMAND Pool made the proposal for the first Sv2 extension called PPLNS with Job Declaration

The spec for this extension currently lives here: https://github.com/demand-open-source/share-accounting-ext/blob/master/extension.md

There's a discussion around it here: https://delvingbitcoin.org/t/pplns-with-job-declaration/1099

We should consider eventually migrating it into this repository

cc @Fi3