Add support for model and schema image fields

[ellmetha]

Description

Marten presently supports file model fields and schema fields, which respectively allow (i) to attach files to model records and (ii) to process / validate uploaded files as part of schemas. Another common need of web applications is to handle image uploading and validation. Ideally, it should be possible to attach images to models using an image model field:

class BlogPost < Marten::Model
  field :id, :big_int, primary_key: true, auto: true
  field :title, :string, max_size: 100
  field :header_image, :image # uses the default media file storage
  field :other_image, :image, storage: my_custome_storage
end

It should also be possible to define image fields as part of schemas:

class BlogPostSchema < Marten::Model
  field :title, :string, max_size: 100
  field :header_image, :image
end

This comes with an additional set of requirements: it becomes necessary to validate the type of files being attached to model records or processed by schemas in order to ensure that only image files are allowed. Non-image files should result in validation errors. Validating that a file is indeed an image could be achieved by leveraging some external image processing shards; and a few candidates could be considered:

• fastimage
• crymagick
• pluto

Regardless of the chosen shard, this shouldn't become a "hard" dependency of Marten: Marten should work as expected even if the chosen shard is not installed, but using image model / schema fields wouldn't not be possible.