Add support for creating tables and columns with comments

[andrew-farries]

Update the create table and add column operations so that they support adding Postgres comments:

• create table: comments can be added to the table itself and to each of its columns.
• add column: a comment can be added to the column.

A create table migration that includes a comment on the table itself and on one of its columns looks like this:

{
  "name": "12_create_employees_table",
  "operations": [
    {
      "create_table": {
        "name": "employees",
        "comment": "This is a comment for the employees table",
        "columns": [
          {
            "name": "id",
            "type": "serial",
            "pk": true
          },
          {
            "name": "role",
            "type": "varchar(255)",
            "comment": "This is a comment for the role column"
          }
        ]
      }
    }
  ]
}

and an add column migration that includes a comment looks like this:

{
  "name": "30_add_column_simple_up",
  "operations": [
    {
      "add_column": {
        "table": "people",
        "up": "'temporary-description'",
        "column": {
          "name": "description",
          "type": "varchar(255)",
          "nullable": false,
          "comment": "This is a comment for the description column"
        }
      }
    }
  ]
}

This allows new tables and columns to be created with comments.

Deletion and modification of comments should still be performed with a raw SQL migration. Until we see a use case that requires versioned modification/removal of comments, these operations are best performed directly on the base table with raw SQL.