include/context: Add method to log directly to a context

include/context: Add method to log directly to a context

When logging messages to a context, a process needs to write messages
to a logfile and then add the logfile to a context. This makes the
control flow rather complicated because temporary files need to be
created, written to, and removed correctly when errors occur.
This commit adds the foundry_context_log() method that allows the
caller to append messages directly to a logfile.