Add tgswitch which allows to easily manage different terragrunt
versions for different projects.
Tested it locally on a NixOS machine:
```
❯ tgswitch --help
Usage: tgswitch [-hv] [-b value] [parameters ...]
-b, --bin=value Custom binary path. For example:
/Users/username/bin/terragrunt
-h, --help displays help message
-v, --version displays the version of tgswitch
Supply the terragrunt version as an argument, or choose from a menu
```
