gRPC support

This is a probably a very big task, since JS Workers don't seem to have an existing library or a nice way to do it. There are some existing crates like [tonic](https://github.com/hyperium/tonic) which support gRPC but these likely wouldn't work without a lot of changes. gRPC also has streams, which some platforms don't support while still supporting unary requests (possibly for similar reasons that might prevent Workers from supporting them). [gRPC-web](https://github.com/grpc/grpc-web) might be easier, but do not currently support streams.

Another possible issue is Cloudflare's support for gRPC since HTTP/2 isn't just enabled for everything by default and Workers might not currently support HTTP/2(?).

- [Related Hacker News thread](https://news.ycombinator.com/item?id=24650696)

No repository files or tests are named. Start by investigating tonic, grpc-web, and the stated Workers HTTP/2 and streaming constraints; the issue needs a scoped implementation plan before coding.

Written by the indexing model from the issue text.