text: Title: Add support for inline conditionals for query building
  Body: ## Problem
  
  <!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] -->
  
  Currently it is annoying to write update mutation data with conditional inputs, for example:
  
  ```py
  data: UserUpdateInput = {}
  if name:
      data["name"] = name
  
  if colour != 'red':
      data["colour"] = colour
  
  await User.prisma().update(
      data=data,
      where={"id": user_id},
  )
  ```
  
  ## Suggested solution
  
  <!-- A clear and concise description of what you want to happen. -->
  
  We should support something like this:
  
  ```py
  await User.prisma().update(
      data={
          'name': name if name else prisma.omit,
          'colour': colour if colour != 'red' else prisma.omit,
      },
      where={
          'id': user_id,
      },
  )
  ```
  
  We may also want to export the `prisma.omit` special value to the client instance as well to avoid an extra import.
  