Support :path option to import_sdl

[bruce]

This adds support for the import_sdl/1 and import_sdl/2 notation macros to load SDL from a file location while supporting recompilation via @external_resource.

This also adds support for more flexible arguments to the macro (supporting function calls, etc)

Example

Given the following SDL:

type Query {
  hello: String!
}

defmodule MyAppWeb.Schema do
  use Absinthe.Schema

  # Option 1: Embed it
  import_sdl """
  type Query {
    hello: String!
  }
  """

  # Option 2: Load it (from a file, etc) using custom logic:
  import_sdl File.read!("path/to/schema.graphql")

  # Option 3: Load it from a file, supporting recompilation on-change:
  import_sdl path: "schema.graphql"

  # Option 4: The same as above, but programmatically determining the path:
  import_sdl path: Path.join(:code.priv_dir(:myapp), "schema.graphql")

  # Not shown: Use decorations to apply resolvers, etc
end