Introduce mjpEncoder plugin architecture

Add a new mjpEncoder plugin type mirroring the existing mjpDecoder pattern.
Encoders serialize an mjSpec + mjModel to an mjResource for a given format.

New API functions:
- mjp_registerEncoder: globally register an encoder
- mjp_defaultEncoder: zero-initialize an encoder struct
- mjp_findEncoder: look up an encoder by filename extension or content type

The mjfEncode callback takes (mjSpec*, mjModel*, mjVFS*, mjResource*) and
returns 0 on success. Writing to mjResource keeps symmetry with the decoder
reading from mjResource and leaves the door open for writable resource providers.

PiperOrigin-RevId: 889187898
Change-Id: I180771b2255b91dea188ac5e2cdc3a8f0fb85364

include/mujoco/mjplugin.h

@@ -89,6 +89,19 @@ struct mjpDecoder {
 };
 typedef struct mjpDecoder mjpDecoder;
 
+//---------------------------------- Encoder -------------------------------------------------------
+
+typedef int (*mjfEncode)(const mjSpec* s, const mjModel* m, const mjVFS* vfs,
+                         mjResource* resource);
+
+struct mjpEncoder {
+  const char* content_type;
+  const char* extension;
+  mjfEncode encode;
+  mjfCloseResource close_resource;
+};
+typedef struct mjpEncoder mjpEncoder;
+
 //---------------------------------- Plugins -------------------------------------------------------
 
 typedef enum mjtPluginCapabilityBit_ {