Approves a pending request, optionally changing the amount (and, for a new budget,
the budget to create). Only admin / budgets_manage users, who can approve their own
requests too.

Effects

• more_funds on a funded ("Con saldo") budget, without periods or with
  period_scope: current_period: tops the budget up from the company's free balance,
  like POST /v1/budgets/{id}/top_up. If the balance doesn't cover it, it fails like
  the top-up (budgets/insufficient_company_funds with unreserved_cents) and the
  request stays pending.
• more_funds with period_scope: next_periods: raises the budget's
  base_amount_cents from the next renewal on. It moves no balance today, so it never
  fails for lack of funds; next_period_shortfall_cents over 0 warns that the
  company's free balance wouldn't cover renewing it with the new base today.
• more_funds on a tracking ("Solo control") budget: raises its
  allocated_amount_cents (or its base_amount_cents with next_periods). No
  balance moves.
• new_budget: creates the budget like POST /v1/budgets, with the request as
  default (funded, the requester as Encargado, the amount asked for) and budget
  overriding it. The request's budget_id points to it from then on.

Any failure leaves the request pending.