Move CustomKernelThunk into its own file

[copybara-service[bot]]

Move CustomKernelThunk into its own file

CustomKernelThunk is currently declared in kernel_thunk.h and this change moves it into its own file custom_kernel_thunk.h. The same is done for the implementation (kernel_thunk.cc to custom_kernel_thunk.cc and the tests).

It also moves the shared logic (PrintBufferContents function) into its own file and adds tests.