Implement 'create index' migrations

[andrew-farries]

Add support for create index migrations. A create index migration looks like this:

{
  "name": "10_create_index",
  "operations": [
    {
      "create_index": {
        "table": "fruits",
        "columns": [
          "name"
        ]
      }
    }
  ]
}

• On Start the index is added to the underlying table.
• On Rollback the index is removed from the underlying table.
• Complete is a no-op.

There are more options that could be supported here, but are not included in this PR including:

• Different index types besides the default (btree)
• UNIQUE
• NULLS FIRST / LAST
• NULLS DISTINCT
• Partial indexes