[FEATURE]: Add findUnique to relational queries

### Describe what you want

Hello 👋,

Relational queries are pushing the DX of Drizzle closer than Prisma, I love it!

In order to close the gap, it would be great to have [findUnique](https://www.prisma.io/docs/reference/api-reference/prisma-client-reference/#findunique) implemented. It's basically like `findFirst` but restricted to the filter of at least one ID/PK/Unique column (https://www.prisma.io/docs/reference/api-reference/prisma-client-reference/#filter-on-non-unique-fields-with-userwhereuniqueinput).