Add a sql2pgroll package to convert SQL to pgroll migrations

[andrew-farries]

Add a sql2pgroll package to convert SQL to pgroll migrations.

Add a (hidden for now) pgroll sql command that uses the package to convert SQL strings on the command line to pgroll migrations.

The sql2pgroll package is incomplete, with almost all SQL falling back to conversion using raw SQL migrations. Only some CREATE TABLE statements and the ALTER TABLE ... ALTER COLUMN ... SET NOT NULL statement are currently handled.

$ pgroll sql "create table foo(a serial primary key, b text unique)" 

[
  {
    "create_table": {
      "columns": [
        {
          "name": "a",
          "pk": true,
          "type": "serial"
        },
        {
          "name": "b",
          "nullable": true,
          "type": "text",
          "unique": true
        }
      ],
      "name": "foo"
    }
  }
]

Part of #504

[andrew-farries]

Issue tracking the work remaining on sql2pgroll: #504

[exekias]

awesome feature! looking forward to seing it progressing.

On the command name, I'm unsure about sql but don't have a better proposal, so let's go for it and see how it goes