feature: support streaming data in place of strings

[karenetheridge]

Some strings, e.g. from an HTTP request or response body, might be huge, and copying them around might be useless anyway if they are not validated (e.g. just their existence is needed, and keywords like pattern are not always applied to them). In these cases it could be helpful to pass them around in the data payload as a scalar reference or a streamed object (Mojo::Content::Single).

Therefore the keywords that are valid against strings should recognize these types:

• type
• const
• enum
• maxLength (use $content->body_size)
• minLength ("")
• pattern
• uniqueItems