Move fee_receiver address to the Reserve Config

[DaSichuan]

Moves the fee receiver from the ReserveLiquidity struct to the ReserveConfig struct
so that it can be modified by the update-reserve-config command, and because it
makes more sense for it to be there.

[DaSichuan]

Tbh this is a pretty ugly change. Some parts of the code are pretty annoying. For example,
there are functions that take in the ReserveConfig as a param (which holds the fee receiver pubkey), but also need the fee receiver Keypair (to create the fee receiver account), so that gets passed into the function as well, so the reserve pubkey is sort of passed in twice, which is pretty annoying. Maybe theres a nicer way to do this that I'm missing?

Also I think we'll need to change some front end deserialization code since I changed where the fee receiver address lives to keep things logically grouped together.

[nope-finance]

this seems wrong shouldn't it be the thing you passed in?

[DaSichuan]

For adding a new reserve, we just make a new one, because we also just make the fee receiver account. in this command.
The thing passed in is for updating the reserve