Add support for custom session cookie serializers

[cconstantin]

This is to ease integration with existing applications, in particular rails.

Current behaviour is maintained (serialize to ETF), but support for JSON is added.

[scrogson]

My original PR (#72) included this feature, but it appears it was removed before merging. @josevalim do you remember why? I'm a 👍 on allowing a serializer to be configured.

@cconstantin the only thing I'm not a big fan of is the namespaces for the serializers. Since a serializer could be used for things other than cookies I think Plug.Serializer(s).JSON may be more appropriate.

[josevalim]

@scrogson it was removed because it was added as part of the encryptor while it should be a responsibility of the cookie store (as in this pull request). :)

[scrogson]

Ah! Makes sense.

[cconstantin]

@josevalim anything you'd change with this PR then? I'm not sure what to expect about how/when feedback is provided. :)

[josevalim]

I am on holidays so I don't have a lot of time for reviewing and merging code lately but I plan to get back to it soon-ish. :)

[cconstantin]

Enjoy your time off. :)

[josevalim]

Let's merge this but I would like to propose some changes.

1. Let's only expect atoms (i.e. modules) as argument to serializer. Strings/binaries won't be supported
2. The default serializer can be :external_term_format and we don't need to implement a module. We can just check if the value is :external_term_format and call term_to_binary / binary_to_term directly.
3. Let's not provide a default JSON serializer. Instead, people can simply pass Poison and it will work because Poison exports decode/encode. We expect those functions to return {:ok, _}. Anything else will be ignored.

The goal is to make the code simpler and remove abstractions. We need to rebase and improve the docs on the exact API a serializer is required to implement.

[cconstantin]

@josevalim updated the code. Any other suggestions? Thanks.

[josevalim]

Beautiful ❤️ 💚 💙 💛 💜