[Feature] Add "nulls [not] distinct" option to Postgres index config

### Is this your first time submitting a feature request?

- [X] I have read the [expectations for open source contributors](https://docs.getdbt.com/docs/contributing/oss-expectations)
- [X] I have searched the existing issues, and I could not find an existing issue for this feature
- [X] I am requesting a straightforward extension of existing dbt-postgres functionality, rather than a Big Idea better suited to a discussion

### Describe the feature

Postgres indexes have [many configurable parameters](https://www.postgresql.org/docs/current/sql-createindex.html#parameters), while dbt-postgres's built-in `indexes` model configuration only supports a couple: `columns`, `type`, and `unique`. Previously, [a feature request](https://github.com/dbt-labs/dbt-adapters/issues/677) was made to support configurable `name`s. I would like to have support for the `NULLS [NOT] DISTINCT` option:

> Specifies whether for a unique index, null values should be considered distinct (not equal). The default is that they are distinct, so that a unique index could contain multiple null values in a column.

The implementation could have `nulls_distinct=true` as an arg with a default value, mirroring current (Postgres' default) behavior.

### Describe alternatives you've considered

I've used [`dbt_utils.generate_surrogate_key`](https://github.com/dbt-labs/dbt-utils?tab=readme-ov-file#generate_surrogate_key-source), which handles nulls in a configurable manner. However, I've run into performance issues resulting from indexing on hashed values, rather than a nicely sorted composite index.

I've also implemented a custom macro for creating postgres indexes with additional parameters, but its usage is a bit hacky, and I don't _really_ want to maintain that logic on my own.

### Who will this benefit?

Users of dbt-postgres who need easy access to more of the functionality implemented by Postgres indexes. Specifically, in cases of unique indexes for which one of the columns has distinct, nullable values.

### Are you interested in contributing this feature?

definitely maybe :)

### Anything else?

_No response_