refactor: make SelfUpdate a step

[SteveLauC]

Standards checklist:

• The PR title is descriptive.
• I have read CONTRIBUTING.md
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
• Optional: I have tested the code myself

For new steps

• Optional: Topgrade skips this step where needed
• Optional: The --dry-run option works with this step
• Optional: The --yes option works with this step if it is supported by
  the underlying command

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.

What does this PR do

Make self_update a step

$ cargo b --features self-update -q

$ ./target/debug/topgrade  --only self_update

── 12:14:01 - Self update ──────────────────────────────────────────────────────
Checking target-arch... x86_64-unknown-linux-gnu
Checking current version... v10.0.2
Checking latest released version... v12.0.2
New release found! v10.0.2 --> v12.0.2
New release is *NOT* compatible

topgrade release status:
  * Current exe: "/tmp/topgrade/target/debug/topgrade"
  * New exe release: "topgrade-v12.0.2-x86_64-unknown-linux-gnu.tar.gz"
  * New exe download url: "https://api.github.com/repos/topgrade-rs/topgrade/releases/assets/118492348"

The new release will be downloaded/extracted and the existing binary will be replaced.
Downloading...
[00:00:01] [========================================] 5.54MiB/5.54MiB (0s) Done
Extracting archive... Done
Replacing binary file... Done

Topgrade upgraded to 12.0.2: