bazel: Added a rule to copy a file

This is unnecessarily basic, but it can sometimes be useful to
copy a file from one repository to another.

Signed-off-by: SlyMarbo <the.sly.marbo@googlemail.com>

bazel/copy_file.bzl

@@ -0,0 +1,27 @@
+# Copyright 2022 The Firefly Authors.
+#
+# Use of this source code is governed by a BSD 3-clause
+# license that can be found in the LICENSE file.
+
+def _copy_file_impl(ctx):
+    outfile = ctx.actions.declare_file(ctx.label.name)
+    ctx.actions.run_shell(
+        inputs = [ctx.file.src],
+        outputs = [outfile],
+        command = "cp %s %s" % (ctx.file.src.path, outfile.path),
+    )
+
+    return [DefaultInfo(
+        files = depset([outfile]),
+    )]
+
+copy_file = rule(
+    implementation = _copy_file_impl,
+    attrs = {
+        "src": attr.label(
+            doc = "Label of the file to copy.",
+            allow_single_file = True,
+            mandatory = True,
+        ),
+    },
+)