Pretty SDL export

[Sytten]

Description of the feature

It would be nice to have a "pretty" export mode for the SDL. Similar to what you would get from graphql codegen.
What we have currently is pretty close, it is just a matter of adding a few options:

struct SDLExportOptions {
  with_newline: bool // This adds a newline between each element
  with_sorted_fields: bool // This sorts the fields in each type in alphabetical order
}

We use the schema as part of the review process so it's nice when it is easier to read.