feat(x/minfee): extract go module

[rootulp]

Draft PR until I come back from OOO

Motivation

As part of the refactor to a multiplexed app, we want a mechanism to avoid importing state machine modules (e.g. x/minfee) once per state machine version. We want this because each time a state machine module is imported by the multiplexer, it registers proto types in the proto registry and interfaces in the Cosmos SDK interface registry. When we import the same state machine module more than once, we hit duplicate proto registry errors (same for Cosmos SDK interface registry).

This PR is the first of many that spin out a distinct Go module per state machine module. Expect subsequent PRs for x/blob, x/signal, etc.

Note: if we didn't do this for the multiplexed app, it is likely that we would do this as part of an update to new Cosmos SDK versions. For example Cosmos SDK v0.52.x spun out Go modules for all the state machine modules.

Testing

I did this on my fork and verified that we can:

1. Publish multiple Go modules from the same repo (i.e. celestia-app, and x/minfee)
2. From celestia-app import x/minfee. See here

Ref:

• https://github.com/rootulp/celestia-app/releases/tag/v3.0.0
• https://github.com/rootulp/celestia-app/releases/tag/x%2Fminfee%2Fv1.1.0