Introduce trilinear flex parametrization.

These flexes use only 24 DOFs (3 per vertex of the bounding box), while colliding with the full high resolution mesh.

On an 8x8x8 cube, the performance using DOFs at all vertices is

```
 Simulation time      : 18.74 s
 Steps per second     : 533
 Realtime factor      : 0.53 x
 Time per step        : 1874.4 µs

 Contacts per step    : 114.88
 Constraints per step : 3322.51
 Degrees of freedom   : 1536
```

With the new implementation, it is the following:

```
 Simulation time      : 1.82 s
 Steps per second     : 5507
 Realtime factor      : 5.51 x
 Time per step        : 181.6 µs

 Contacts per step    : 38.84
 Constraints per step : 155.36
 Degrees of freedom   : 24
```

PiperOrigin-RevId: 721008829
Change-Id: I5266c4166f1abff7e681646c73da5579e859bff2

Author: quagla

dm_control/mjcf/schema.xml

@@ -1148,6 +1148,7 @@
             <attribute name="euler" type="array" array_type="float" array_size="3"/>
             <attribute name="origin" type="array" array_type="float" array_size="3"/>
             <attribute name="dim" type="int" required="true"/>
+            <attribute name="dof" type="string"/>
           </attributes>
           <children>
             <element name="plugin" repeated="true" namespace="flexcomp">
@@ -1543,6 +1544,7 @@
                 <attribute name="euler" type="array" array_type="float" array_size="3"/>
                 <attribute name="origin" type="array" array_type="float" array_size="3"/>
                 <attribute name="dim" type="int" required="true"/>
+                <attribute name="dof" type="string"/>
               </attributes>
               <children>
                 <element name="plugin" repeated="true" namespace="flexcomp">
@@ -1740,6 +1742,7 @@
             <attribute name="vertex" type="array" array_type="float" array_size="5"/>
             <attribute name="element" type="float"/>
             <attribute name="texcoord" type="float"/>
+            <attribute name="node" type="array" array_type="float" array_size="3"/>
           </attributes>
           <children>
             <element name="edge" repeated="true">