Add Liquidity Mining support for Isolated pools

[green-jay]

LM farms can be created currently only for the Omnipool subpools via a feature specific implementation @ https://github.com/galacticcouncil/HydraDX-node/tree/master/pallets/omnipool-liquidity-mining

Task is to analyse which other pools would be suitable for farms and create either a generic LM connector or extend the current implementation with Isolated (XYK) pool support.

Goal is to be able to create LM farms on top of Isolated pools.

[green-jay]

closed in #798