Allow the indexer definition function to be async

[fracek]

We should allow the indexer definition function to be async to allow users to fetch any required data from an external service. For example, users may store configuration in an external vault and fetch the values at runtime.

export default async function (runtimeConfig: ApibaraRuntimeConfig) {
  return defineIndexer(StarknetStream)({
    // ...
  });
}

Required changes

• Make the createIndexer helper async.
• Make sure we propagate that change across callers.

Testing

We should update the example to use the async definition. The example should call await (for example an http call to httpbin.org) somewhere before returning the indexer.