Add `update` subcommand to pull the latest CLI release version locally

Target component

• CLI
• SDK
• Something else
• Not sure

Overview

Looking at the list of command options with the oxide CLI, it doesn't look like there's a way to update in-place:

Commands:
  experimental
  certificate
  disk
  floating-ip
  group
  image
  instance
  project
  current-user
  ping          Ping API
  policy
  snapshot
  system
  silo
  ip-pool
  user
  utilization   Fetch resource utilization for user's current silo
  vpc
  auth          Login, logout, and get the status of your authentication.
  api           Makes an authenticated HTTP request to the Oxide API and prints the response.
  docs          Generate CLI docs in JSON format
  version       Prints version information about the CLI.
  completion    Generate shell completion scripts for Oxide CLI commands.
  help          Print this message or the help of the given subcommand(s)

And no subcommand option to do so either with the version command:

izzyfee@Elizabeths-MBP /opt/oxide-cli-x86_64-apple-darwin$ oxide version --help                                                                                                                                                                           
Prints version information about the CLI.

Usage: oxide version

Options:
  -h, --help  Print help

Implementation details

While it's easy enough to grab the latest release from https://github.com/oxidecomputer/oxide.rs/releases, it might be nice to be able to do something like oxide version update --latest or oxide version list to see a list of versions available wherein one could then specify the version they want to go to (including downgrading if needed), e.g. oxide version update --target <version_num>.

Anything else you would like to add?

No response

Start with the CLI's version command and its current help output, then review the release versions available from the linked GitHub releases page. Clarify whether the feature should support only --latest or also listing and targeting versions, including downgrades. Done should let a locally installed CLI update to the requested latest release.