Adds estimatevault RPC

[Jouzo]

What kind of PR is this?:

/kind feature

What this PR does / why we need it:

Adds estimatevault RPC.
This RPC takes an amount of collateral tokens and an amount of loan tokens to estimate the resulting vault

./src/defi-cli -testnet estimatevault '["1000.0000000@DFI"]' '["0.6599999@GOOGL"]'
{
  "collateralValue": 3003.06883000,
  "loanValue": 1928.78370776,
  "informativeRatio": 155.69754233,
  "collateralRatio": 156
}

Which issue(s) does this PR fixes?:

Fixes part of #796