Connect to stdin/stdout?

### Is your feature request related to a problem? Please describe.

To be able to build a buildkit frontend, we have to connect to a GRPC server over stdin/stdout

https://github.com/moby/buildkit/blob/914e64243dbaa2ccb8798d732c6f7bafbb708eaa/frontend/gateway/grpcclient/client.go#L390-L392
https://github.com/moby/buildkit/blob/914e64243dbaa2ccb8798d732c6f7bafbb708eaa/frontend/gateway/grpcclient/client.go#L406-L408

However, I don't see how this should work with `@grpc/grpc-js`. Is there an easy way to pass streams to a Client / Channel so they can be used instead of an url?

### Describe the solution you'd like
A way to pass streams / an option to use stdin&stdout for the client as a channel

### Describe alternatives you've considered
Extending a Channel and passing this to a client, but there doesn't seem an easy way to pass an input and output stream to