text: Title: Add support for full text search for PostgreSQL
  Body: [https://www.prisma.io/docs/concepts/components/prisma-client/full-text-search](https://www.prisma.io/docs/concepts/components/prisma-client/full-text-search)
  
  ## Suggested solution
  
  Something like the following, `search` should be added to `StringFilter` and must be an instance of `String`.
  
  Should be noted that I'm not stuck on `String` being the name.
  
  ```py
  # NOTE: actual API is still TODO
  from prisma.querying import String
  
  await client.post.find_first(
      where={
          'content': {
              'search': String.contains('cat', 'dog'),
          },
      },
  )
  await client.post.find_first(
      where={
          'content': {
              # for anything we don't explicitly support
              'search': String.raw('fox \| cat'),
          },
      },
  )
  ```