Generate examples for UUID formatted strings

[zorbash]

This is a continuation of #266 (comment)
we should generate valid examples for as many formats as possible.

The current behaviour regarding UUIDs is illustrated in the following example:

defmodule SomeSchema do
  require OpenApiSpex

  alias OpenApiSpex.{Schema, Cast}

  OpenApiSpex.schema(%{
    description: "some schema",
    type: :object,
    properties: %{
      id: %Schema{
        type: :string,
        format: :uuid
      }
    }
  })
end

OpenApiSpex.Cast.cast(SomeSchema.schema, OpenApiSpex.Schema.example(SomeSchema))

# => {:ok, %SomeSchema{id: ""}}

The generated example contains an invalid UUID and cast/2 doesn't validate it at all returning a success tuple.