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

wasm/codegen/generators/constants.py

@@ -46,15 +46,18 @@ _SKIPPED_PLUGIN_FUNCTIONS: tuple[str, ...] = (
     "mjc_getSDF",
     "mjc_gradient",
     "mjp_defaultDecoder",
+    "mjp_defaultEncoder",
     "mjp_defaultPlugin",
     "mjp_defaultResourceProvider",
     "mjp_findDecoder",
+    "mjp_findEncoder",
     "mjp_getPlugin",
     "mjp_getPluginAtSlot",
     "mjp_getResourceProvider",
     "mjp_getResourceProviderAtSlot",
     "mjp_pluginCount",
     "mjp_registerDecoder",
+    "mjp_registerEncoder",
     "mjp_registerPlugin",
     "mjp_registerResourceProvider",
     "mjp_resourceProviderCount",
@@ -74,6 +77,8 @@ _SKIPPED_CLASS_METHODS: tuple[str, ...] = (
     "mj_deleteModel",
     "mj_deleteSpec",
     "mj_deleteVFS",
+    "mj_encode",
+    "mj_parse",  # TODO(manevi): Bind this function.
     "mj_loadModel",
     "mj_loadXML",
     "mj_makeData",