Add support for grpc-web proxy for better browser clients integration

[zvolin]

Summary

I'm working on a wasm support for celestia-app grpc client. The core idea is to use a grpc-web client and a grpc-web proxy, which will translate requests from browser to regular grpc requests. Everything seems to work using the go grpcwebproxy server as a bridge between a browser and celestia-app.

This issue proposes integrating a grpc-web proxy directly into the celestia-app node.

Problem Definition

While running celestia-node or submitting transactions using bare grpc connection to celestia-app, a user must select a validator node it trusts. Browser clients will instead need to select the grpc-web proxy that they trust, and they may not be able to control which validator is selected under the hood by grpc-web proxy server. Integrating the proxy directly in celestia-app node would allow browser clients to connect to validators directly and keep a single trust point instead of relying on centralized 3rd party hosted proxies.

Proposal

Expose a new grpc-web proxy endpoint in celestia-app (may be optional, no hard opinion about defaults) that will proxy traffic to grpc endpoint. The proxy server I used for testing in lumina is written in Go so maybe it can be used as a module.