Add raw option for attributes

[HassanAkbar]

Add raw tag for attributes to keep the raw xml as value of that attribute.

fixes #90

[HassanAkbar]

@ronaldtse by looking at this comment it seems like a case for raw option but in this case we need to define it for the current model.
Do you think it would be beneficial to add raw option for the model as well? may be at the root, something like below (it might need some adjusting)

class RichText  < Lutaml::Model::Serializable
  attribute :raw_string, :string

  xml do
    root 'RichText', raw: true

    map_content to: :raw_string
  end
end

class Docid < Lutaml::Model::Serializable
  attribute :raw_string, RichText
  
  xml do
    root 'docid'

    map_content to: :raw_string
  end
end

[ronaldtse]

@HassanAkbar I think there are two reasons why @opoudjis and @andrew2net want an enhancement here:

1. Using raw because the underlying XML structure is complex or unknown, and it is time consuming to parse the underlying XML structure using Lutaml::Model. This is handled via raw.
2. Text models for rich text is complicated, and actually things like attribute :sup... is totally useless for the user, because you won't be "selecting every bold text instance" but you just want to deal with it as XML. This should be supported as commented by @andrew2net . I don't know what is the best API to do so, and we should support it.

[ronaldtse]

@HassanAkbar let's merge this first, but is it possible to obtain the full raw string instead of serializing it again? I just don't want to modify something that is meant to be "raw".

[HassanAkbar]

@ronaldtse the conversion in lutaml-model has 2 steps

1. Data is parsed using the adapter and saved in the adapter document class (at this point we have no idea which classes have which mappings defined)
2. Mappings are used to convert that data to ruby object

The problem is that when we are parsing the data we do not have access to the mappings so we have no idea which content to read as raw string and which to parse. So that is why we are converting the parsed data back to XML string when applying mappings.

[ronaldtse]

Can’t the parser return us the raw XML string at a certain node?

[HassanAkbar]

@ronaldtse It will require some changes in the adapters but I think it will work something like

value = if child.text?
          child.text
        elsif attr&.raw?
          child.to_xml
        else
          parse_element(child, attr&.type || klass, format)
        end

I'll open a separate PR for this.

[ronaldtse]

@HassanAkbar can we rebase and merge this? Thanks!