feat: update indexing to risingwave index instead of postgres

[MattiasMTS]

Currently, the indexing is done by the dbt-postgres adapter due to inheritance. Risingwave's indexing supports this but it has additional configuration that is not configurable if you build e.g. MWs with indexes.

Docs
https://docs.risingwave.com/docs/current/indexes/

Mainly, the two INCLUDE and DISTRIBUTED BY should be configurable in the config of a dbt model.

Also, right now you can also configure the type (i.e. index method) and unique as indicated by https://docs.getdbt.com/reference/resource-configs/postgres-configs#indexes. However, RisingWave doesn't support these changes and if you try to change them it doesn't make any impact.