feat: add operation messages via RAISE NOTICE

[benjie]

Fixes #12 (cc @demianuco)

PostGraphile will subscribe to NOTICE events from PostgreSQL during mutations, and any notice with error code OPMSG (operation message) will be added to the mutation payload messages field.

Important:

• This feature only applies to mutations
• Any PL/pgSQL function may raise these notices
• Additional information can be added to the OPMSG by passing a JSON value to the detail field
• RAISE NOTICE may have message length limits; it's your responsibility to colour inside the lines
• Clients must be configured (via client_min_messages) to receive NOTICE messages; this is PostgreSQL's default configuration.
• If the server is configured (via log_min_messages) to log NOTICEs then, well, the NOTICEs will be logged; PostgreSQL's default configuration does not log NOTICE.

Example:

raise notice 
  '2 + 2 is %, minus 1 that''s %; quick maths.',
  (2 + 2),
  (2 + 2 - 1)
using
  errcode = 'OPMSG',
  detail = json_build_object(
    'level', 'info',
    'path', array_to_json(ARRAY['noticePath']),
    'anything_else', 'can_go_here'
  )::text;

[benjie]

Rebased on #17 (after merge to master) to make the diff smaller and more focussed.

[mathroc]

I don’t understand how notices are filtered, it looks like code different from OPMSG would be processed too, am I missing something ?

[benjie]

👀 I was so tired that day. I wasn't confident merging because I knew I'd overlooked something but couldn't figure out what. Thanks for spotting @mathroc! Added test and fixed.