Add 'BondOperation'

[kov]

While the original design had several types of bonds (like CDB, Tesouro Direto), I think we should have a more generic Bonds which has the type of bond as a detail rather than a top-level type. We'll need information such as

• term
• any combination of:
  • fixed rate (think Tesouro Prefixado)
  • floating rate (think Tesouro Selic)
  • indexed rate (think Tesouro IPCA+, which also has a fixed rate btw)
• liquidity
• coupon
• coupon frequency