Add response middlewares to the network queryers

[cideM]

Context

The Content API team has a feature in its GraphQL API that requires both custom directives to work (already fixed) and extensions support in GQL responses (this PR + one in the gateway library fork + one in our GQL gateway). Specifically, we want to forward the extensions.relations key that Content API returns for queries carrying the @capiRelationMap directive.

Problem

Both network queryers parse the full downstream response and then decode only data into the receiver. SingleRequestQueryer.Query (queryerNetwork.go):

result := map[string]interface{}{}
if err = json.Unmarshal(response, &result); err != nil {
    return err
}
// ...
if err = decoder.Decode(result["data"]); err != nil {
    return err
}
return q.queryer.ExtractErrors(result)

Notice that extensions is ignored and thrown away. MultiOpQueryer.Query (queryerMultiOp.go) has the same problem.

Solution

There is a request middleware in the repo already, NetworkMiddleware func(*http.Request) error, but there's no equivalent for responses, so we'll add one:

type ResponseMiddleware func(ctx context.Context, response map[string]interface{}) error

type QueryerWithResponseMiddlewares interface {
    WithResponseMiddlewares(wares []ResponseMiddleware) Queryer
}

The middlewares run after the response is parsed and before data is decoded, with the full top-level object (data, errors, extensions, ...). Returning an error aborts the query with that error, like a NetworkMiddleware does on the request side.

The library makes no assumptions about what a middleware does with the response. Our use is to collect extensions per operation in the GQL gateway (https://github.com/amboss-mededu/graphql-gateway/pull/695), but it is equally usable for logging, metrics or tracing on the response side.

Tests cover the single request queryer (JSON and multipart, error propagation) and the multi-op queryer (each bundled query sees its own entry).

This change is written to be upstreamable to nautilus/graphql as is.

[greenmato]

LGTM (see comment on https://github.com/amboss-mededu/graphql-gateway/pull/695#pullrequestreview-5207108619)

[cideM]

nautilus#47