Make trading fees configurable for each pool

[jakubgregus]

Basilisk-node/pallets/xyk/src/lib.rs

Line 94 in 9624ace

type GetExchangeFee: Get<(u32, u32)>;

[mrq1911]

i think we can make this change with minimal impact on the API and existing code by introducing new storage with a fee per pool (current constant will be the default) and extrinsic set_fee callable by the specified origin.

The question is who (the origin) will be able to set the fee? referendum?