Making `k6/experimental/grpc` module part of the k6 core

[olegbespalov]

What?

In the https://github.com/grafana/k6/releases/tag/v0.45.0 we introduced the new experimental module of the GRPC, which supports streaming.

This issue is for tracking the path of the extension; here, we could give some updates on its status.

Why?

The long-term plan is to replace the existing k6/net/grpc with this new extension once we prove that it satisfies the customers' needs and is stable enough.