feat(cargo-shuttle): Add upgrade subcommand to run install script

[supleed2]

Description of change

Adds a function that spawns a child process running the install command, using conditional compilation based on target family (unix vs windows). Closes #1840

How has this been tested? (if applicable)

• Ran cargo run --bin cargo-shuttle -- upgrade on Windows
  • With latest cargo-shuttle installed: got the expected message that my cargo-shuttle version is latest from binstall
  • Without cargo-shuttle, with cargo-binstall: got output from binstall that binary was downloaded from GitHub
  • Without cargo-shuttle and cargo-binstall: directly downloads binary which works (I realise we didn't add the prompts to the powershell version)
• Ran cargo run --bin cargo-shuttle -- upgrade on Ubuntu
  • With latest cargo-shuttle installed: got the expected message that my cargo-shuttle version is up-to-date from the bash script
  • Without cargo-shuttle, with cargo-binstall: got output from binstall that binary was downloaded from GitHub
  • Without cargo-shuttle and cargo-binstall: got prompts to download binary directly which works, and to install from source via cargo which also works

[jonaro00]

Can you also test the interactivity of the subprocess, i.e. that the y/n prompts work as intended when you don't have cargo-binstall?

[supleed2]

Thanks for the reminder, just ran through the other scenarios and updated my comment.