Add ability to log from the tracing layer directly rather than use XPTI

[callumfare]

This adds the ability to use the tracing layer with a logger rather than relying on XPTI. The motivation for this is to provide an equivalent to SYCL_PI_TRACE when we remove PI from the SYCL runtime. urtrace and sycl-trace will still be supported.

[pbalcer]

Also, we might want to document this, right now the tracing layer documentation only lists the xpti entry points.