GSoC 2021 Proposal: Add regexp-parser() (Xiaoyu Qiu)

Using regular expressions to parse messages is very common and useful. Currently, syslog ng supports extracting fields (or name-value pairs) in its filter expressions. However, those are primarily meant for filtering and don't store extracted groups by default (despite we can manually add store-matches flags option). To make up for this deficiency, this project is to develop a new parser type, here I call it regexp-parser(). The motivation behind regexp-parser() is to remove the need to use filters to extract name-value pairs from content, which has traditionally been done by parsers. And these names can be used as user-defined macros for subsequent filtering or other types of message processing.