ActiveStorage alternative?

Nobody has claimed this yet.

It would be very cool if Avram could have, either built in or as a separate shard, a good alternative to ActiveStorage. I did a little playing around and thought this would make a good API for it:

**config/storage.cr**
```crystal
require "avram/storage/local"
require "avram/storage/s3"

# Here is where you configure how Avram stores files
Avram::Storage.configure do |storage|
# Store files on a local hard disk
disk_adapter = Avram::Storage::DiskAdapter.new(root: "/path/to/root")\
storage.attach_adapter(:local, disk_adapter)

# Store files remotely using Amazon S3
s3_adapter = Avram::Storage::S3Adapter.new(
access_key_id: "",
secret_key: "",
region: "",
bucket: ""
)
storage.attach_adapter(:s3, s3_adapter)

# Set the default adapater based on the environment.
if Lucky::Env.production?
storage.set_default_adapter(:s3)
else
storage.set_default_adapter(:local)
end
end
```

**src/models/user.cr**
```crystal
class User < BaseModel
include Avram::Attachable
include Authentic::PasswordAuthenticatable

table :users do
column email : String
column encrypted_password : String

# One-to-one attachment
has_one_attached :avatar

# Set an adapter explicitly
has_one_attached :avatar, adapter: :s3

# One-to-many attachment
has_many_attached :avatars
end
end
```

As for how to implement it we could probably do something similar to what ActiveStorage does on the back end and just create a polymorphic join table called `avram_attachments` or something that references the name of the class and the path the file is stored at (relative to the adapter). The module `Avram::Attachable` could keep track of the classes that contain attachments.

Start with the proposed config/storage.cr and src/models/user.cr examples, then review Avram's existing model and migration conventions. The scope needs agreement around local and S3 adapters, attachment associations, and the polymorphic avram_attachments table before implementation can be considered complete.

Written by the indexing model from the issue text.