Support for edge runtimes

[timostamm]

We plan to tackle support for edge runtimes, and our high-level plan is as follows:

1. Identify the status quo for various runtimes. We should know exactly what works and what doesn't and at the end of it have a test suite that runs on local and actual to prove the scenarios.
2. Prioritizing Connect/gRPC-web protocol and unary, identify the common work that can be done across runtimes. This will probably be supporting WinterCG spec.
3. Document the support coverage for various runtimes and their reasons. Release any support packages that may be needed.

We're starting with support for Cloudflare workers and Vercel.

For Cloudflare, #550 tracks serving RPCs, and #577 tracks support for clients. For serving RPCs on Cloudflare, being able to to pass env and ctx parameters to service implementations is an important feature - interceptor capabilities with an augmented HandlerContext (see #527) is therefore also an important feature for Cloudflare support.

For Vercel, we would like to support the new Next.js App Router with Connect RPCs (#542). Ideally, we want to support Connect RPCs in plain Vercel Edge functions as well.