Support schema, and table descriptions

[leondutoit]

https://www.postgresql.org/docs/current/sql-comment.html

"There is presently no security mechanism for viewing comments: any user connected to a database can see all the comments for objects in that database. For shared objects such as databases, roles, and tablespaces, comments are stored globally so any user connected to any database in the cluster can see all the comments for shared objects. Therefore, don't put security-critical information in comments."

https://stackoverflow.com/questions/7426205/sqlite-adding-comment-on-descriptions-to-tables-and-columns

[leondutoit]

Need to consider the tradeoffs of using the COMMENT system and sqlite DDL, versus offering a meta table as part of core functionality, similar to audit.

[leondutoit]

So that listing a schema could result in getting:

{
  "metadata": {},
  "tables": {
      "table1": {}, 
      "table2": {}
   }
}

Behaviour:

• the top-level keys metadata and tables are always present, even if their values are empty
• when a table is created it is added as a key in the value of tables
• clients can edit the values, not the keys

[leondutoit]

Docs on "field size" suggest the postgres limit to be 1GB: https://www.postgresql.org/docs/current/limits.html