feat: Support LP token swaps on UniswapV2

[kayibal]

Support selling and buying LP tokens on the UniswapV2Executor.

Corresponding simulation issue:
propeller-heads/tycho-simulation#129

The Executor contract should use the same steps the simulation logic uses:

• Sell LP token
  • check is the input token is an LP token
  • burn the LP token in exchange for two reserves
  • sell one of the reserves on the pool to provide a single reserve to the caller
• Buy LP token
  • check if the output token is an LP token
  • take one of the reserves and swap it to the second reserve
    • This will require a method that uses the current spot price to determine how much of one reserve should be swapped for the other.
  • then deposit both reserves to receive the LP token

Most of this logic is already implemented in the UniswapV2Router02 [docs] [code] in the following methods: addLiquidity, removeLiquidity, addLiquidityETH, removeLiquidityETH. We can perhaps copy these methods into our UniswapV2Executor to avoid some calls to an external contract (and maybe save gas).

On the encoding side, nothing needs to change.