[VEN-2030]: add flash swap liquidation operator

[kkirka]

This PR contains a contract to perform liquidations on Venus using flash loans (for in-kind liquidations) and flash swaps (for cross-token liquidations) on PancakeSwap V3.

The following kinds of liquidations are supported:

1. In-kind liquidation (liquidate BUSD borrow seizing vBUSD) – uses the pool from the provided path to source the liquidity from, flash-loans from this pool using flash() and performs the liquidation.
2. Cross-token liquidation:
   a. Using a single pool (e.g., liquidate BUSD seizing vBNB) – flash-swaps the collateral token to the borrowed token using swap(), performs the liquidation and repays the flash swap using collateral tokens directly.
   b. Liquidating along the path (BUSD-TokenB-TokenC-...-CollateralToken, e.g. liquidate BUSD seizing vADA, using BUSD/WBNB pool as a liquidity source) – flash-swaps the path's TokenB to the borrowed token using swap(), performs the liquidation, swaps collateral tokens to TokenB along the remainder of the path, and repays the flash swap using the TokenB.

Notes:

• The script for choosing the optimized path is out of the scope of this PR.
• This PR will likely not be merged into this repository, I propose we create a new repo for such contracts (and possibly scripts working with such contracts).
• Unit-tests and more fork tests are to be developed.