Add nix output for web-grpc client bundled as a single JS file

[palas]

Changelog

- description: |
    Add nix output that produces a bundle with a web-grpc client for `cardano-rpc`
  type:
  - feature
  projects:
  - cardano-rpc

Context

In order to use cardano-grpc from JS in the browser, we need a library that can be embedded in vanilla JS. This PR introduces a nix output that produces a self-contained bundle JS file from the .proto file.

How to trust this PR

It is basically reviewing the nix, and testing it.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[carbolymer]

👌🏻