[pkg/stanza] Add 'strip_ansi_escape_codes' operator

[gjasny]

Description

We use the filelog receiver to ingest Artifactory logs. Some but not all of them contain ANSI color sequences. This makes regex creation pretty uncomfortable because both: log field separators and the ANSI color codes contain [ characters.

I was first thinking about using the add operator and removing the regex within the EXPR(). But I could not find any suitable regex function which works within that expression.

I could imagine that I'm not the only one who would like to get rid of colors. Therefore I believe that an easy to use operator enhances readability of the pipeline and is less error-prone than a (potentially) hand-crafted regex.

Testing

Added unit tests and tested locally against Artifactory log files.

[atoulme]

This is a neat enhancement, thank you. I have reviewed the code and approve it, the codeowners have last word on accepting it.