Add `install.sh`

Users should be able to run `curl -fsSL https://raw.githubusercontent.com/Flagsmith/flagsmith-cli/main/install.sh | sh` to install the CLI in their environment.

### Acceptance criteria

The following should work:

```sh
$ curl -fsSL https://raw.githubusercontent.com/Flagsmith/flagsmith-cli/main/install.sh | sh
$ flagsmith   # CLI output
```