mgr/ActivePyModules: refactor perf schema section management to use RAII

Refactor get_perf_schema_python() and get_unlabeled_perf_schema_python() to use Formatter::ObjectSection and Formatter::ArraySection RAII helpers instead of manual close_section() calls. Manual closes have have led to crashes when close_section() calls were left unguarded, for example when an OSD with no perf counters had it's perf counters queried.

Fixes: https://tracker.ceph.com/issues/74693
Signed-off-by: stzuraski898 <steven.zuraski@ibm.com>