src.ctf.fs: use C++ value wrappers in queries

src.ctf.fs: use C++ value wrappers in queries

 - Pass the query parameters as a bt2::ConstMapValue
 - Return the query result as a bt2::Value::Shared
 - Communicate errors via exceptions
 - Use std::string instead of GString to build the metadata text

Change-Id: If75da1efc1accb5c42dd98d830ab9abf2ffff932
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8165
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12271
Tested-by: jenkins <jenkins@lttng.org>