Add useJSONContent option for express and sveltekit

[CahidArda]

When adding nextjs-pages integration, we realized that the QStash verification would fail because the bodies don't match. After inspection, here is what we saw:

• triggerFirstInvocation which runs in the client.trigger or in the first workflow endpoint invocation adds content-type: application/json by default.
• If you send this body: { "test" : "message" }, QStash uses the whole string when creating signature.
• But nextjs-pages receives {"test":"message"} because of the content-type header.

To fix the issue, we switched to publish in triggerFirstInvocation.

As a result of this however, some frameworks didn't work.

• In express we only support content-type: application/json currently.
• Sveltekit would get empty string as body when content-type wasn't application/json.

[linear]

DX-1470 workflow-js fix framework integrations

[joschan21]

I find this very confusing because we cannot assume anyone is familiar with our internal workings, they will not know what triggerFirstInvocationis. To me a much more friendly approach would be directly specifying headers that are then forwarded, those can include the content type but are not limited

[CahidArda]

useJSONContent is not part of the options we allow in serve methods for frameworks (see PublicServeOptions). So it won't be visible to users unless they they import serve from the route like import { serve } from "@upstash/workflow"

The reason we need this parameter is because we used to have publishJSON in client.trigger. this didn't work because publishJSON adds content-type: application/json, which breaks some frameworks. So we used publish. But some frameworks still need the content type to be application/json. sveltekit is one of them.

If you have a better alternative to the current implementation we can change it

[CahidArda]

removed useJSONContent from the public API