Consider execution-time fee specification schema with centralized IntegratorFeeStores

[alnoki]

Related: #336

Consider allowing fee rate to be specified at execution time, that is, during the function call in which a trade is processed

For example, rather than passing only integrator address, pass integrator and fee_rate_bps, so that integrators can specify the fees.

This schema will allow, for example:

• Sophisticated traders who trade using SDKs to pay no taker fees
• Granular control for frontend partners to specify fees based on business logic like perks, giveaways, etc.
• Aggregators to invoke Econia trades with 0 taker fees, rather than, for example, trying to rebate fees back to users through a FeeStore

If the specified integrator address doesn't have a IntegratorFeeStore, or if the fee rate is specified as 0, then no fees will be charged

[alnoki]

• Have integrator fee stores be a unit of account in a global registry, like a table for each market
• Then when fees get deposited to an integrator, their table entry just needs to be updated
  • The fee amount for each integrator accrued could be an aggregator, to further enable parallelism
• At the user level, emit events or otherwise track a list of all the markets that an integrator has registered under (index by user and by market)
• Coins could go into a global vault, rather than moved between counterparties and integrator in each trade