Change the .sql headers so they are treated as comments by SQL clients

[tatiana]

trafficstars

Context At the moment, Astro (0.5.1) uses frontmatter blocks at the top of the SQL files to decide on the configuration to use to run the queries.

The consequence is that SQL clients cannot run those SQL files. When trying to comment on these headers using standard SQL comments (--), Astro is not able to interpret them.

Acceptance criteria

• Allow users to declare configuration headers in SQL files used by Astro
• Allow SQL clients to interpret SQL files used by Astro

• Ask @mag3141592 (bug reporter) to review the PR