Allow aos::Sha256() to take a file path

This enables writing some tests where we want to conveniently compare
that the contents of two files are identical or haven't changed from
some known-good baseline.

Change-Id: I7f62fb1b3fffb7feafd34dd776e8a4981820b7d6
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/BUILD b/aos/BUILD
index f1cb6ba..acef762 100644
--- a/aos/BUILD
+++ b/aos/BUILD
@@ -809,7 +809,18 @@
     target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
+        "//aos/util:file",
         "@boringssl//:crypto",
         "@com_google_absl//absl/types:span",
     ],
 )
+
+cc_test(
+    name = "sha256_test",
+    srcs = ["sha256_test.cc"],
+    deps = [
+        ":sha256",
+        "//aos/testing:googletest",
+        "//aos/testing:tmpdir",
+    ],
+)