chore: add import formatting to make fmt/go (#21453)

Modifies `make fmt/go` to also use https://github.com/daixiang0/gci to format our imports to a standard format.

It introduces a new shell script to do the formatting so that our formatting tools are in one place.

scripts/format_go_file.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+
+# This script formats Go file(s) with our project-specific configuration.
+
+# Usage: format_go_file <file>...
+
+set -euo pipefail
+
+if [[ "$#" -lt 1 ]]; then
+	echo "Usage: $0 <file>..."
+	exit 1
+fi
+
+go run mvdan.cc/gofumpt@v0.8.0 -w -l "${@}"
+go run github.com/daixiang0/gci@v0.13.7 write -s standard -s default -s "Prefix(github.com/coder,cdr.dev/)" "${@}"