Add support for upgrading GitHub CLI Extensions

Hi there!

This PR adds support for upgrading GitHub CLI extensions, which are custom commands for the GitHub CLI that anyone can create and use. Once installed locally, they can be upgraded from their respective upstream repos. See https://docs.github.com/en/github-cli/github-cli/using-github-cli-extensions

This fixes #868

Since this is my very first PR to this repo, please do let me know if things should be done differently.

Thanks.

Standards checklist:

[x] The PR title is descriptive.
[x] The code compiles (cargo build)
[x] The code passes rustfmt (cargo fmt)
[x] The code passes clippy (cargo clippy)
[x] The code passes tests (cargo test)
[x] Optional: I have tested the code myself
- [x] I also tested that Topgrade skips the step where needed