Stargate mock support

[DariuszDepta]

This PR defines Stargate trait, that is a hook for mocking stargate messages and queries in tests. Initial implementation provides two stargate "keepers": StargateFailing and StargateAccepting. StargateFailing always fails while processing stargate messages or queries - this is the default behaviour in App. StargateAccepting silently processes all stargate messages and queries, the returned values are just default AppResult and default Binary respectively.

Implementing custom Stargate keeper allows to customize the way the stargate messages and queries are processed. This way users may adjust this behaviour to their needs while testing contracts with stargate messages or queries.

Detailed usage examples are provided in test cases in MultiTest. General way to customize stargate keeper in tests may look like this:

1. Implement your stargate keeper:

struct MyStargateKeeper;

impl Stargate for MyStargateKeeper {
    fn execute<ExecC, QueryC>(
        &self,
        api: &dyn Api,
        storage: &mut dyn Storage,
        router: &dyn CosmosRouter<ExecC = ExecC, QueryC = QueryC>,
        block: &BlockInfo,
        sender: Addr,
        type_url: String,
        value: Binary,
    ) -> AnyResult<AppResponse>
    where
        ExecC: Debug + Clone + PartialEq + JsonSchema + DeserializeOwned + 'static,
        QueryC: CustomQuery + DeserializeOwned + 'static,
    {
        // place your custom processing here
    }

    fn query(
        &self,
        api: &dyn Api,
        storage: &dyn Storage,
        querier: &dyn Querier,
        block: &BlockInfo,
        path: String,
        data: Binary,
    ) -> AnyResult<Binary> {
        // place your custom processing here
    }
}

2. Provide your stargate keeper to App using AppBuilder:

let my_stargate_keeper = MyStargateKeeper;
let app_builder = AppBuilder::default();
let mut app = app_builder.with_stargate(my_stargate_keeper).build(no_init);

3. Test your contract:

// place your testing logic here, below is shown an example taken from internal tests

let code = app.store_code(stargate::contract());
let owner_addr = app.api().addr_make("owner");
let contract_addr = app
    .instantiate_contract(code, owner_addr.clone(), &Empty {}, &[], "tauri", None)
    .unwrap();
app.execute_contract(owner_addr, contract_addr, &Empty {}, &[]).unwrap();

---

closes #88
closes #40
closes #37
closes #64

[chipshort]

LGTM