mtmd: use sha256 for input hashing (#27274)

* mtmd: use sha256 for input hashing

* void conflict with boringssl

vendor/hash/hash.cpp

@@ -0,0 +1,23 @@
+#include "hash.h"
+
+extern "C" {
+#include "sha256/sha256.h"
+}
+
+static std::string to_hex(const unsigned char * digest, size_t len) {
+    static const char hex[] = "0123456789abcdef";
+
+    std::string out;
+    out.reserve(2*len);
+    for (size_t i = 0; i < len; ++i) {
+        out += hex[digest[i] >> 4];
+        out += hex[digest[i] & 0xf];
+    }
+    return out;
+}
+
+std::string hash_sha256_hex(const void * data, size_t len) {
+    unsigned char digest[SHA256_DIGEST_SIZE];
+    sha256_hash(digest, (const unsigned char *) data, len);
+    return to_hex(digest, SHA256_DIGEST_SIZE);
+}