Create a Graphql API to combine fees and margins estimation APIs

[edd]

The API implemented in #1898 provides clients with an overview of the fees, but not the margin requirements. After vegaprotocol/specs#311 is written, we'll have a way to get an estimate for the margin for an order. The fees API should be able to include this to provide a complete cost estimate to the client, given an order.

Note: this is written from the perspective of a REST or GraphQL client. Margin and Fees in the core are quite separate, and it is fine if the GRPC endpoints reflect this. However for REST & GraphQL we want a single endpoint that, given an order, can return a breakdown of the estimated cost.

Given we already have the fees breakdown on the market object, we can calculate the fees part ourself. So the bulk of this task is actually working out how to get the margin estimate out to the client.

Needs: #1898

[jeremyletang]

This depends on #2299 to be done first.