Add CLI install scripts

[florisvdg]

Here are the CLI install script modifications I did for the CircleCI orb to support versioning.

Now that we want to use it for NPM too (as a Git submodule) as well as future (CI) integrations I've created a repo for it.

It will also allow for static links with a commit hash, making a curl-pipe-to-sh a little bit more trustworthy, e.g.:
curl https://raw.githubusercontent.com/secrethub/install-scripts/217358495fc965dbd42099a606f9852101476dc1/cli_unix.sh | sh

TO-DO

• Add support for version pinning in Windows. We basically need f345d7b, but then in the PowerShell script.
• Add test for Windows. See ./.github/workflows/test.yml.
• (Optional) Don't install Windows version if it already exists. 33e9dc4, but then in PowerShell.

[florisvdg]

This is no longer necessary for NPM.