MINOR: log: introduce "log-steps" proxy keyword

MINOR: log: introduce "log-steps" proxy keyword

For now it is only available for proxies with frontend capability because
log-steps are only evaluated under sess_log() or strm_log() which
essentially focus on the frontend side when it comes to log settings so
it's better to keep it this way for better consistency, at least for now.

For now the setting does nothing (it is not considered during runtime),
it will be implemented and documented in upcoming commits.