bug: Compute Engine Diagonal is returning the wrong type

In `GPJax/gpjax/kernels.base.py`, the return type of the `diagonal` method in `AbstractKernel` should change from
```
def diagonal(self, x: Num[Array, "N D"]) -> Float[Array, " N"]:
```
to
```
def diagonal(self, x: Num[Array, "N D"]) -> LinearOperator:
```
to both match the return type of `gram()` and the expected return type in `AbstractKernelComputation`.