luks: Show the JSON information for LUKSv2

Extract the full information for version 2, which is JSON format. Show
this with the 'luks info' command.

Use the mmc12 disk to check this.

Require the JSON for LUKS.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/luks.rst

@@ -106,6 +106,35 @@ Display LUKS header information for a LUKS2 partition::
     Label:
     Checksum alg:   sha256
 
+    JSON metadata (12288 bytes):
+    {
+      "keyslots": {
+        "0": {
+          "type": "luks2",
+          "key_size": 64,
+          "kdf": {
+            "type": "argon2id",
+            "time": 6,
+            "memory": 1048576,
+            "cpus": 4,
+            ...
+          },
+          ...
+        }
+      },
+      "tokens": {},
+      "segments": {
+        "0": {
+          "type": "crypt",
+          "offset": "16777216",
+          "encryption": "aes-xts-plain64",
+          ...
+        }
+      },
+      "digests": { ... },
+      "config": { ... }
+    }
+
 Display LUKS header information for a LUKS1 partition::
 
     => luks info mmc 1:1