Add joint-to-joint position error function to support relative position constraint (#900)

Summary:
Pull Request resolved: #900

This diff adds a new `JointToJointPositionErrorFunction` that constrains the relative position between two joints. Unlike the existing `PositionErrorFunction` which targets world-space positions, this error function expresses constraints in a reference joint's local coordinate frame.

**Motivation:**
This is useful for constraints where you want to control the relative position between two body parts, such as:
- Keeping a hand at a specific position relative to the torso
- Maintaining specific spatial relationships between joints that should move together
- Expressing constraints that are naturally defined in a local reference frame rather than world space

**Implementation:**
The error is computed as:
```
error = w * ||R_ref^T * (p_src - p_ref) - target||²
```

where:
- `p_src = T_src * sourceOffset` is the source point in world coordinates
- `p_ref = T_ref * referenceOffset` is the reference point in world coordinates
- `R_ref` is the rotation of the reference joint
- `target` is the desired relative position in the reference frame

The gradient computation handles three contributions:
1. Source joint chain: positive contribution from `d(srcWorldPos)/dq`
2. Reference joint chain: negative contribution from `d(refWorldPos)/dq`
3. Reference rotation: contribution from `d(R_ref^T)/dq` affecting the frame transformation

Reviewed By: cdtwigg

Differential Revision: D89738249

fbshipit-source-id: d11b64a95db4c149ae39484f0bf0a614fecd95c2

Author: clarkdykang

cmake/build_variables.bzl

@@ -227,6 +227,7 @@ character_solver_public_headers = [
     "character_solver/gauss_newton_solver_qr.h",
     "character_solver/height_error_function.h",
     "character_solver/joint_to_joint_distance_error_function.h",
+    "character_solver/joint_to_joint_position_error_function.h",
     "character_solver/limit_error_function.h",
     "character_solver/model_parameters_error_function.h",
     "character_solver/normal_error_function.h",
@@ -258,6 +259,7 @@ character_solver_sources = [
     "character_solver/gauss_newton_solver_qr.cpp",
     "character_solver/height_error_function.cpp",
     "character_solver/joint_to_joint_distance_error_function.cpp",
+    "character_solver/joint_to_joint_position_error_function.cpp",
     "character_solver/limit_error_function.cpp",
     "character_solver/model_parameters_error_function.cpp",
     "character_solver/normal_error_function.cpp",