refactor(#2964): timestamp conversion is broken in adapters

[tenthe]

Purpose

This PR includes:

• Refactoring of the timestamp transformation rule in adapters
• Fix for the timestamp rule related to string regex expressions.
• Added tests for timestamp transformation rules with Unix timestamps in seconds and milliseconds (currently commented out, to be covered in a separate PR as this is already quite large).

Remarks

PR introduces (a) breaking change(s): no

PR introduces (a) deprecation(s): no