Add pluggable resource writing to MuJoCo

Extend mjpResourceProvider with an optional write callback (write)
so that mj_encode, mj_saveXML, and mj_saveModel can write to any
registered provider.

PiperOrigin-RevId: 945202741
Change-Id: I37903425260932e555f4a8c2392c4ff8c2e6cc06

include/mujoco/mjplugin.h

@@ -58,6 +58,10 @@ typedef int (*mjfUnmountResource)(mjResource* resource);
 // returns < 0 if the resource is older than the given timestamp
 typedef int (*mjfResourceModified)(const mjResource* resource, const char* timestamp);
 
+// callback for writing bytes to a resource
+// return number of bytes written, return -1 if error
+typedef mjtSize (*mjfWriteResource)(mjResource* resource, const void* buffer, mjtSize nbytes);
+
 // struct describing a single resource provider
 typedef struct mjpResourceProvider {
   const char* prefix;               // prefix for match against a resource name
@@ -67,6 +71,7 @@ typedef struct mjpResourceProvider {
   mjfMountResource mount;           // mounting callback (optional)
   mjfUnmountResource unmount;       // unmounting callback (optional)
   mjfResourceModified modified;     // resource modified callback (optional)
+  mjfWriteResource write;           // writing callback (optional)
   void* data;                       // opaque data pointer (resource invariant)
 } mjpResourceProvider;