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

[zw963]

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 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