sysutils/fluent-bit: Add option LUA_FILTER

From https://docs.fluentbit.io/manual/pipeline/filters/lua:

The Lua filter allows you to modify the incoming records (even split
one record into multiple records) using custom Lua scripts.

The option depends on luajit-devel, because at least version 2.1.0 is
required: https://github.com/fluent/fluent-bit/pull/7286/files.

Keep the option disabled for now. We may consider enabling it by default
in the future to match what upstream does:
https://docs.fluentbit.io/manual/installation/sources/build-and-install.

Also, add a small do-test target for the new option.

PR: 252296
Approved by: portmgr (maintainer timeout)
Sponsored by: trivago N.V.
Sponsored by: Klara, Inc.