feat(extio): Add EXTIO command data structures

Reverse engineered from a real EXTIO device, open-io
project as further reference

src/main/extio/cmd.c

@@ -0,0 +1,16 @@
+#include "cmd.h"
+
+uint8_t extio_cmd_checksum(const struct extio_cmd_write *write)
+{
+    const uint8_t *data;
+    uint32_t checksum;
+
+    data = (const uint8_t *) write;
+
+    checksum = 0;
+    checksum += data[0];
+    checksum += data[1];
+    checksum += data[2];
+
+    return (uint8_t) (checksum & 0x7F);
+}