SNOW-802269 Added regexp_extract,signum,substring_index,collect_list

[sfc-gh-sjayabalan]

1) Added regexp_extract,signum,substring_index,collect_list to functions.scala . 2) Added test cases for the same

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[sfc-gh-sjayabalan]

Please change the PR title to SNOW-802269 Added regexp_extract,signum,substring_index,collect_list It should start with the Jira number

Modified the pr title.Please verify