fix: use union-based cast to resolve const-correctness without warnings

The previous cast approach triggered -Wcast-qual warnings in CI.
This implementation uses a union to safely convert const to non-const
pointer, which is a standard C technique that avoids compiler warnings.

How it works:
- Data array is const (preventing modification)
- Union allows type-punning between const and non-const pointers
- Accessing via .ptr member gives non-const pointer for struct assignment
- No undefined behavior as per C standard union rules

This maintains const-correctness of the underlying data while being
compatible with struct s2n_blob's non-const data pointer member.

Author: akshatsinha0

utils/s2n_blob.h

@@ -69,6 +69,10 @@ int S2N_RESULT_MUST_USE s2n_blob_slice(const struct s2n_blob *b, struct s2n_blob
     struct s2n_blob name = { 0 };                        \
     RESULT_GUARD_POSIX(s2n_blob_init(&name, name##_buf, name##_requested_size))
 
-#define S2N_BLOB_LABEL(name, str)             \
-    static const uint8_t name##_data[] = str; \
-    const struct s2n_blob name = { .data = (uint8_t *) name##_data, .size = sizeof(name##_data) - 1 };
+#define S2N_BLOB_LABEL(name, str)                                    \
+    static const uint8_t name##_data[] = str;                        \
+    static union {                                                   \
+        const uint8_t *const_ptr;                                    \
+        uint8_t *ptr;                                                \
+    } name##_union = { .const_ptr = name##_data };                   \
+    const struct s2n_blob name = { .data = name##_union.ptr, .size = sizeof(name##_data) - 1 };