feature(adjoint): add custom internal run function with new custom_vjp arguments to provide hook into gradient computation for custom vjp calculation.

Author: Gregory Roberts

CHANGELOG.md

@@ -50,6 +50,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added more RF-specific mode characteristics to `MicrowaveModeData`, including propagation constants (alpha, beta, gamma), phase/group velocities, wave impedance, and automatic mode classification with configurable polarization thresholds in `MicrowaveModeSpec`.
 - Introduce `tidy3d.rf` namespace to consolidate all RF classes.
 - Added support for custom colormaps in `plot_field`.
+- Added `custom_vjp` and new custom run functions that provide hooks into adjoint for custom gradient calculations.
 
 ### Breaking Changes
 - Edge singularity correction at PEC and lossy metal edges defaults to `True`.