Start the API observability project: Create a Session object to put the logging hooks

Federico Mena Quinterorequested to merge
federico/librsvg:observable-api into main

Design doc is in devel-docs/api_observability.rst.

This MR is just for the initial refactoring to make rsvg_log!() take a session argument. That session object is what will take care of the logging since the time an API entry point until the end of an SvgHandle's lifetime.

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