Linux installation script

### Feature Description

Bash script for automatic `cloud-sql-proxy` installation/update with latest version auto pulling from github repo.

### Sample code

```bash
#!/usr/bin/env bash
set -euo pipefail

# Cloud SQL Auth Proxy installer/updater
# - Installs to /usr/local/bin/cloud-sql-proxy
# - By default installs the latest GitHub release
#
# Usage:
# sudo ./install-cloud-sql-proxy.sh
#
# Optional env overrides:
# VERSION=v2.20.0 sudo ./install-cloud-sql-proxy.sh
# INSTALL_DIR=/usr/local/bin sudo ./install-cloud-sql-proxy.sh

REPO="GoogleCloudPlatform/cloud-sql-proxy"
INSTALL_DIR="${INSTALL_DIR:-/usr/local/bin}"
BIN_NAME="cloud-sql-proxy"

need_cmd() {
command -v "$1" >/dev/null 2>&1 || {
echo "Error: missing required command: $1" >&2
exit 1
}
}

need_cmd curl
need_cmd uname
need_cmd mktemp
need_cmd install

if [[ "${EUID:-$(id -u)}" -ne 0 ]]; then
echo "Error: please run as root (use sudo)." >&2
exit 1
fi

ARCH="$(uname -m)"
case "$ARCH" in
x86_64|amd64) PLATFORM="linux.amd64" ;;
aarch64|arm64) PLATFORM="linux.arm64" ;;
i386|i686) PLATFORM="linux.386" ;;
*)
echo "Error: unsupported architecture: $ARCH" >&2
exit 1
;;
esac

# Get latest version tag from GitHub unless VERSION is provided.
if [[ -n "${VERSION:-}" ]]; then
TAG="$VERSION"
else
# Using GitHub Releases "latest" endpoint; parse tag_name without jq.
# Example response includes: "tag_name": "v2.20.0"
TAG="$(
curl -fsSL "https://api.github.com/repos/${REPO}/releases/latest" \
| sed -nE 's/.*"tag_name"[[:space:]]*:[[:space:]]*"([^"]+)".*/\1/p' \
| head -n1
)"
if [[ -z "$TAG" ]]; then
echo "Error: could not determine latest release tag from GitHub." >&2
exit 1
fi
fi

BASE_URL="https://storage.googleapis.com/cloud-sql-connectors/cloud-sql-proxy/${TAG}"
FILE_NAME="${BIN_NAME}.${PLATFORM}"
DOWNLOAD_URL="${BASE_URL}/${FILE_NAME}"

TMPDIR="$(mktemp -d)"
cleanup() { rm -rf "$TMPDIR"; }
trap cleanup EXIT

echo "Installing ${BIN_NAME} ${TAG} (${PLATFORM})"
echo "Downloading: ${DOWNLOAD_URL}"

curl -fsSL "$DOWNLOAD_URL" -o "${TMPDIR}/${BIN_NAME}"
chmod +x "${TMPDIR}/${BIN_NAME}"

# Install atomically with correct permissions
install -m 0755 "${TMPDIR}/${BIN_NAME}" "${INSTALL_DIR}/${BIN_NAME}"

echo "Installed to: ${INSTALL_DIR}/${BIN_NAME}"
echo -n "Installed binary reports: "
if "${INSTALL_DIR}/${BIN_NAME}" --version >/dev/null 2>&1; then
"${INSTALL_DIR}/${BIN_NAME}" --version
else
echo "(version flag not available; run '${BIN_NAME} --help' to verify)"
fi
```

### Alternatives Considered

For now you need to do it manually, of course it's easy, but I guess this script can save a little bit of time for somebody.

### Additional Details

Tested on `Ubuntu 24.04.3 LTS`.
I didn't create a PR because this may be just a script available to download/use only from documentation site.