[TRACKING] Rules syntax enhancements (backward-compatible)

[leogr]

Motivation

This issue tracks all the efforts aimed to introduce new constructs (in a backward compatible fashion) to address the most common user requests, as per the Falco 1.0.0 roadmap.

N.B. Other issues will be opened for backward-incompatible changes to the language syntax.

Additional context

• [Feature] Supporting field transformers in filtering language  libs#1789
• [New Features]: Extend Falco's rules expression language to additional "SQL" like filtering options common in Big Data frameworks  #2403

Tasks

Beta Give feedback

1. New transformer: join(<list>, <sep>) libs#1925
   kind/feature +1
2. New transformer: basename() libs#1927
   kind/feature +1
   
3. New transformer: getopt(<args>, <optstring>) libs#1928
   kind/feature lifecycle/stale +2
4. New transformer: concat(<item1>, <item2>, ...) libs#2025
   kind/feature +1