Add growth and virtual stake to market data

[EVODelavega]

LPs should be able to track their virtual stake and keep track of the growth factor of the market (r in the ELS spec). MarketData already contains a field called LiquidityProviderFeeShares, which contains the ELS, but not the virtual stake. A new field should be added to the LiquidityProviderFeeShare type.
The MarketData type itself should contain a field with the market_growth as a decimal (represented as a string in the protos).