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

doc/changelog.rst

@@ -25,6 +25,10 @@ General
   The polynomial order is defined by the new constant :ref:`mjNPOLY<glNumericSizes>`. A future breaking C-API change
   may unify the linear and higher-order coefficients into a single array.
 
+- Introduced :ref:`mjpEncoder`, the counterpart to :ref:`mjpDecoder` for encoding of :ref:`mjSpec` and :ref:`mjModel` into :ref:`mjResource`.
+
+  - Added :ref:`mj_encode`, :ref:`mjp_registerEncoder`, :ref:`mjp_defaultEncoder`, and :ref:`mjp_findEncoder`.
+
 .. admonition:: Breaking API changes
    :class: attention