Adding liquidity to any pair with only ONE token

Allow users to add liquidity with one of the tokens in any pair and doing the conversion automatically.

Many other top protocols already have this functionality. PancakeSwap cannot stay behind in this.

For example, an user wants to add liquidity to USDC-BUSD pool, but only holds BUSD. This user needs to swap an x% amount of BUSD to USDC MANUALLY and then proceed to adding the desired amount.

This would greatly improve user experience.