Support use a self-defined index type when run create_index in migration.

Nobody has claimed this yet.

What i expected is to create a index like this:

```
CREATE INDEX pgroonga_content_index ON memos USING pgroonga (content);
```

As described by the [tuturial](https://pgroonga.github.io/tutorial/) of that pg extension.

but, because the type of index has been limited to only several type, use create_index is impossible, i have to use execute to do this.

```
execute <<-'HEREDOC'
CREATE INDEX pgroonga_content_index ON memos USING pgroonga (content);
HEREDOC

```

The issue does not name a file or test. Start by tracing the migration create_index API and how it validates index types, then inspect existing migration tests and generated PostgreSQL SQL. Done means create_index can emit a self-defined USING clause such as pgroonga while preserving support for existing index types.

Written by the indexing model from the issue text.