sha256: add sha256_direct_hex() helper

A various places we need a SHA256SUM of something as a string. Let's add
a simple helper for this that does this generically.

src/basic/sha256.h

@@ -11,3 +11,5 @@ int sha256_fd(int fd, uint64_t max_size, uint8_t ret[static SHA256_DIGEST_SIZE])
 int parse_sha256(const char *s, uint8_t ret[static SHA256_DIGEST_SIZE]);
 
 bool sha256_is_valid(const char *s) _pure_;
+
+char* sha256_direct_hex(const void *buffer, size_t sz);