[Assignment-7] Flake + App base

- Add Assignment-7 to flake.nix
- Implement basic framework of app
- Implement proxy subcommand (mostly)
- Implement basics of intermediary subcommand

7-SGX_Hands-on/src/app/util.c

@@ -0,0 +1,23 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "util.h"
+#include "proxy.h"
+#include "intermediary.h"
+
+
+char* BIN_NAME = "SignatureProxy";
+
+void syntax_exit(void) {
+    char* syntax =
+        "SignatureProxy Version 0.0.0\n"
+        "Syntax: %s <command> <arguments>\n"
+        "\n"
+        "Commands:\n"
+        "%s"
+        "\n"
+        "%s";
+
+    printf(syntax, BIN_NAME, intermediary_syntax(), proxy_syntax());
+    exit(1);
+}