Add support for gin indexes

Right now the [docs](https://www.dbml.org/docs/#index-settings) mention several types of indexes but only two are supported (emphasis mine):

> type: type of index (btree, gin, gist, hash depending on DB). **For now, only type btree and hash are accepted.**

Adding better support for index types seems straightforward and would add a lot of value for systems where more than just btree and hash indexes are common.