Suggestion: document high-level "working with custom app-chains"

[dakom]

There are several things that come up in this scenario:

cosmwasm_std:

QueryRequest::Custom

• One example I'm aware of in the wild is Sei https://github.com/sei-protocol/sei-cosmwasm/blob/2d416bfc8ad48ef1190c78bf503bdafb38c6d8d3/packages/sei-cosmwasm/src/querier.rs#L17

• Another is Osmosis: https://github.com/osmosis-labs/osmosis-rust/blob/31fa382125033c37f3adc784a977991c0f85d462/packages/osmosis-std/src/types/osmosis/tokenfactory/v1beta1.rs#L555

QueryRequest::Grpc (related to #127)

cw_multi_test:

Explain the need to mock (since there's no real chain to hit), with examples for both Custom and Grpc. Also highlight how this can be a very good thing since mocking allows testing the contract under extreme scenarios, e.g. the "oracle" can provide prices via proptest and stress-test wild price-swings etc.