gs-plugin-loader: Add sysprof support for profiling jobs

This can be used with a command like:

sysprof-cli --gtk --use-trace-fd -- gnome-software

or from within GNOME Builder, to profile gnome-software. The changes in this commit use the sysprof-capture-3 library to write additional timing data to the capture, showing the details of every plugin job and vfunc call which happen in gnome-software.

This should make it easier to visualise what plugin jobs are happening, and which ones are taking a long time.

This commit adds an optional dependency on sysprof-capture-3.

Signed-off-by: Philip Withnall withnall@endlessm.com