Update the createQuote mutation response type

[mkurapov]

Context

In the CreateQuote method, we are returning a few ILP-specific fields: maxPacketAmount minExchangeRate lowEstimatedExchangeRate highEstimatedExchangeRate. These are used internally only, and unlikely to be useful to any callers of the Admin APIs, so they can be removed from the resulting schema. They become especially unnecessary with the introduction of local payments.

However, it would be useful to start exposing estimatedExchangeRate in the response object, as it can give integrators an easy way to display quote details to their end users.