Add StrategyStellaswapMultiRewardsLP

[cartman-dev]

Add Multi-Reward strategy for Stellaswap w/ unwrapped native support

• Solidity v0.8
• address[][] array for additional reward routes
• All routing via native instead of output
• Use new StratFeeManager
• Does not re-use existing Stella IMasterChef due to it having a v0.6-specific hack
• Not the same as what was used previously for xcDOT-GLMR, which doesn't seem to have made it into this repo