ctf: add ClkClsConfig and use it

ctf: add ClkClsConfig and use it

Different parts of the code in the ctf plugin (in fs-src, lttng-live and
common) use these three values:

 - clock offset (seconds)
 - clock offset (nanoseconds)
 - force origin to be Unix epoch

Gather these three values in a new structure, `ClkClsConfig`, so that
these they can easily be passed around.  This replaces the existing
ctf_fs_metadata_config structure, which contained the exact same thing,
but was specific to the fs component class.

Change-Id: If9b4d88c9fd5eb3352c5de4007dc4343fac485c1
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8228
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>