Made Raw Comments Available to Plugins

[Smithx10]

This PR addresses issue: https://github.com/sqlc-dev/sqlc/issues/3589

I figured passing the raw comment to the plugin author was probably best since metadata seemed to have an opinion around generating go code for queries. Plugin Authors now can implement their own syntax.

It would be nice to be able to capture column level comments to allow for a better user experience for when annotating the column , but I believe we are blocked by: https://github.com/pganalyze/libpg_query/issues/103