mtmd: use sha256 for input hashing (#27274)

* mtmd: use sha256 for input hashing

* void conflict with boringssl

vendor/hash/hash.h

@@ -0,0 +1,9 @@
+#pragma once
+
+// C++ wrapper for the vendored hash functions
+
+#include <cstddef>
+#include <string>
+
+// returns the SHA-256 digest as a lowercase hex string
+std::string hash_sha256_hex(const void * data, size_t len);