[Vault Module]: Refactor vault contract to only manage vault state

[njokuScript]

Description

Currently, the vault contract stores users' funds and interacts directly with the token contracts. We want to make sure that the vault is not making any external calls.

• [ ] Vault should only be responsible for managing vault state(balances, collateralization ratio)
• [ ] Introduce Vault IDs when creating vaults
• [ ] Abstract out Liquidation function from Vault module