feat: Ability to merge routers with different contexts

[ethndotsh]

Describe the feature you'd like to request

The ability to merge routers that have different contexts into one client. For example a Fastify tRPC server + a Next.js tRPC server that have different purposes, but both need to be consumed by the Next.js app which is not possible at the moment.

Describe the solution you'd like to see

KATT mentioned that really only error formatting needs to be the same, so some new merge routers method that merges any two routers as long as their error formatting is the same.

Describe alternate solutions

The only workaround I can think of off the top of my head is calling the secondary server (in my example, the Fastify server) with createServerSideHelpers and then proxying it with a new router inside the Next.js server, although this adds unneeded latency and complexity.

Additional information

#5111

👨‍👧‍👦 Contributing

• 🙋‍♂️ Yes, I'd be down to file a PR implementing this feature!

Funding

• You can sponsor this specific effort via a Polar.sh pledge below
• We receive the pledge once the issue is completed & verified

[ethndotsh]

Just sponsored the issue, hope this can be implemented soon! It is unfortunately blocking the development of my app.

[rjay98]

I'll also sponsor this issue. Primary use-case right now is migrating a Next.js monolith into a separate API server (for other clients to consume). In order to do a piecewise migration, we are experiencing friction when using two separate server endpoints.