Text input, Date input and text area components should support `readonly` in a similar way to `disabled`

[sussexrick]

Currently the only way to specify the readonly attribute for 'Text input' is to add input-readonly="readonly". On 'Textarea' it's textarea-readonly="readonly". On 'Date input' you can add the tag helpers for the individual fields and add readonly="readonly".

In Razor there isn't a way to choose whether to render input-readonly or textarea-readonly dynamically - you have to have two versions of the <govuk-input /> or <govuk-textarea /> tag with and without the attribute, and repeat any logic required to build up child tags.

The implementation of the disabled attribute is better - you can specify a boolean value and have the attribute render or not render. It would be useful if there was a similar readonly=bool attribute across these components. The readonly attribute does not apply to other components.