Named element accessors for `mjx.Model`

### The feature, motivation and pitch

Both `mujoco.MjModel` and `mujoco.MjSpec` have a nice API where their elements can be accessed by name with methods like `mujoco.MjModel.{body,geom,joint,actuator}`. I notice myself constantly missing similar API for `mjx.Model`. For example, most recently, I tried using the playground environment interface together with domain randomization. Unfortunately, [`BraxDomainRandomizationVmapWrapper`](https://github.com/google-deepmind/mujoco_playground/blob/25f570d83fbf13519ca854f08c245b43ebab10bc/mujoco_playground/_src/wrapper.py#L166) only passes `self.mjx_model` into the `randomization_fn`, which makes it hard to access its elements with names.

### Alternatives

I can access the elements with `mjx.Model.bind(mujoco.MjSpec)`, but as the previous example shows, it's quite clumsy to always have to carry both the `mjx.Model` and `mujoco.MjSpec` objects around together. Edit: I can also do `mjx.name2id(name)`, which solves my most immediate issue. It would be much cleaner and consistent to be able to access `mjx.Model` with similar pattern as `mujoco.MjModel`.

### Additional context

Here's a simple test case I wish would pass:
Details

```python
"""Test for `mjx.Model` named element access."""

import textwrap

from absl.testing import absltest
from absl.testing import parameterized
import mujoco
from mujoco import mjx

class MjxModelTests(parameterized.TestCase):
def test_named_element_access(self) -> None:
model = mujoco.MjModel.from_xml_string(textwrap.dedent("""












"""))
modelx = mjx.put_model(model)

self.assertEqual(model.body("body").id, modelx.body("body").id)
self.assertEqual(model.joint("joint").id, modelx.joint("joint").id)
self.assertEqual(model.actuator("motor").id, modelx.actuator("motor").id)
self.assertEqual(model.geom("plane").id, modelx.geom("plane").id)
self.assertEqual(model.geom("box").id, modelx.geom("box").id)

if __name__ == "__main__":
absltest.main()
```

```console
$ python ./mjx_model_test.py
Running tests under Python 3.12.11: /home/user/.venv/bin/python
[ RUN ] UnitTests.test_add
[ FAILED ] UnitTests.test_add
======================================================================
ERROR: test_add (__main__.UnitTests)
UnitTests.test_add
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/user/.venv/lib/python3.12/site-packages/mujoco/mjx/_src/types.py", line 935, in __getattr__
val = getattr(impl_instsance, name)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'ModelJAX' object has no attribute 'body'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/user/mjx_model_test.py", line 30, in test_add
self.assertEqual(model.body("body").id, modelx.body("body").id)
^^^^^^^^^^^
File "/home/user/.venv/lib/python3.12/site-packages/mujoco/mjx/_src/types.py", line 944, in __getattr__
raise AttributeError( # pylint: disable=raise-missing-from
AttributeError: 'Model' object has no attribute 'body'. Did you mean: 'nbody'?

----------------------------------------------------------------------
Ran 1 test in 0.329s

FAILED (errors=1)
```