New transformer: concat(<item1>, <item2>, ...)

[leogr]

Motivation

Please look at this comment for context.

The concat(<item1>, <item2>, ...) transformer concatenates items (either a field or a literal string) given as arguments and returns a new string.

Its output can be utilized in two primary ways:

1. In conditions for filtering.
2. In the output: field of Falco's rule.

One significant use case is concat(fd.rip, ":", fd.rport]) in ("8.8.8.8:53","4.4.4.4:53") as reported by #1981

Feature

Implement the concat(<item1>, <item2>, ...) as described above.
The transformer should return a concatenated string.

Note: Implementing this transformer requires extending the current syntax to allow a variable number of arguments to be accepted by a transformer.

Alternatives

Please look at #1981 for more context on the evaluated alternatives.