Create scripts/install.sh to download and execute the TUI installer based on platforms

[achimnol]

We need a wrapper script like:

#! /bin/bash
OS_TYPE=$(uname -s)
CPU_ARCH=$(uname -m)
SYSTEM="$OS_TYPE $CPU_ARCH"
case "$SYSTEM" in
  "Linux x86_64" )
    PLATFORM="linux-x86_64"
    ;;
  "Linux aarch64" )
    PLATFORM="linux-aarch64"
    ;;
  "Darwin x86_64" )
    PLATFORM="macos-x86_64"
    ;;
  "Darwin arm64" )
    PLATFORM="macos-aarch64"
    ;;
  * )
    echo "Sorry, Backend.AI does not support this platform."
    exit 1
    ;;
esac

# TODO: add GitHub release download
exec "./backendai-install-${PLATFORM}" install "$@"

The script will be linked in other places such as our website, etc.

cc: @kimjmin