Stop quoting column default values

[andrew-farries]

When adding a column to a table (either at creation time or afterwards with an alter table migration), any default value for the column is automatically single quoted.

This makes it easier to define migrations, as the user does not have to remember to add the single quotes in the migration.
For example in this migration:

{
  "name": "28_different_defaults",
  "operations": [
    {
      "create_table": {
        "name": "items",
        "columns": [
          {
            "name": "id",
            "type": "serial",
            "pk": true
          },
          {
            "name": "name",
            "type": "varchar(255)",
            "default": "unnamed"
          }
        ]
      }
    }
  ]
}

the default value for the name column (unnamed) does not need to be quoted as pgroll does that automatically.

However, this automatic quoting causes a problem when assigning a default value of now() to a timestamp column:

{
  "name": "28_different_defaults",
  "operations": [
    {
      "create_table": {
        "name": "items",
        "columns": [
          {
            "name": "id",
            "type": "serial",
            "pk": true
          },
          {
            "name": "created_at",
            "type": "timestamptz",
            "default": "now()"
          }
        ]
      }
    }
  ]
}

when run, this will result in a created_at column with a default value of the time at which the migration was run, eg:

| created_at | timestamp with time zone |  not null default '2023-11-06 08:37:01.203691+00'::timestamp with time zone | 

rather than the desired default:

| created_at | timestamp with time zone |  not null default now() |

This PR removes the automatic quoting of default values so that now(), CURRENT_TIMESTAMP etc can be used correctly as column defaults. This pushes some complexity onto users who now have to single-quote those default values that require it.