Add the ability to introduce repository specific configurations for gh-cli and its extensions

## Feature request description

I would like to propose the addition of a feature that enables the integration of repository-specific configurations into the `pkg/config` package, drawing inspiration from the way Git handles local `.git/config` versus `XDG_CONFIG_HOME` configurations. This enhancement could prove invaluable for various extensions creators who could leverage distinct settings tailored to each repository's requirements.

## Motivation

Consider a chat-op deployment scenario - a good `gh` extension that integrates well with external platforms like Discord or Slack, having the ability to define and commit repository-specific configurations directly within the repository itself can offer significant advantages. For instance, imagine a use case where different repositories need to route communications to specific channels on these platforms. The current `config` package is limited in that the configs are global (AFAIK - please let me know if this is incorrect) which leaves the current approach of providing options each time a command is invoked which can become cumbersome, especially when the values for channels differ across repositories. This is just one scenario where repo specific configs might be useful but I'm sure extension creators can think of creative ways to use this feature 😄

## Implementation

The proposed implementation could involve creating a dedicated configuration file within the repository, such as .ghconfig, where users can define repository-specific settings. This file would be committed alongside other source code and assets.

## Conclusion

Introducing repository-specific configurations to the "gh" library has the potential to enhance the usability, consistency, and flexibility of the tool in scenarios where distinct settings are essential on a per-repository basis. This feature aligns with the natural progression of version control practices and would undoubtedly contribute to streamlining workflows and improving overall efficiency.