Add entry-point to UR which matches `piPluginGetBackendOptions`

[veselypeta]

In intel/llvm/8763 introduced a new entry-point to PI to enable the propagation of compile flags to the device backend compiler.

__SYCL_EXPORT pi_result piPlatformGetBackendOption(pi_platform platform,
                                                   const char *frontend_option,
                                                   const char **backend_option);

We should add a compatible function to UR.

[kbenzie]

First draft of this was called piPluginGetBackendOption but UR didn't carry over the plugin entry point concept so should be a platform entry point instead.