Create `bbctl update bbctl` command

Create a bbctl update bbctl command. This root command will be used in the future for updating the individual application charts but we should treat the bbctl target as a unique case for upgrading the client itself.

Acceptance Criteria

• When run, bbctl should download the latest version of bbctl and replace the existing binary in-place
• Must respect the installation method, e.g. use go install, download a binary, or invoke homebrew if installed that way
• Optionally support downloading a specific version with a --version flag or something.