feat(schema): add budget_extra and budget_prev attributes

those two new attribute will describe:

- the estimated over/under extra consumption of the budget (eg: we expect to
  consume 10 more days)
- the total budget, that will be used for trajectory computations (which is
  the sum of the budget paid by the client and the budget_extra)

related: https://forge.extranet.logilab.fr/logilab/shingouz/-/issues/44
4 jobs for !14 with topic/default/budget-prev-extra in 2 minutes and 35 seconds (queued for 4 seconds)
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  Lint
passed black #440942

00:00:57

passed check-manifest #440941

00:00:50

passed flake8 #440940

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  Tests
passed py3 #440943

00:01:34