Bump to SDK v50 and Celestia App v4

[chatton]

I have begun working on this on this branch in the 01builders repo https://github.com/01builders/celestia-node/tree/sdk-v0.50.x

This branch is a WIP so no need to review it now, I will create a PR against this repo when things are R4R.

Doing this work uncovered a few things that have since been worked on in the celestia-app repo. Specifically around the usage of 2 gRPC connections instead of one.

As a result the final PR should be much smaller than the celesti-app/v4 one.

This work is blocked on some additional changes being introduced into the cosmos-sdk fork

• feat: add block api wiring and block api proxy server for standalone mode cosmos-sdk#455
• feat: block api proxy client/server impl cosmos-sdk#453

And will also require celestia-app changes incorporating these fixes.

[chatton]

feel free to assign me to this!