Implement JsonNode in Layout DTOs

[cfisher-scottlogic]

Background

Layout definitions are currently being transferred to and from the server as raw string values in a single JSON field. Since the layout is defined in a JSON format, we want to embed this as a fully formed node within the request/response.

Work Required

• Convert the definition fields in all layout DTOs and entities from String to JsonNode (import from Jackson databind)
• Annotate the entity field with @Convert to make use of the existing JsonNodeConverter

Notes

• This pattern was implemented for the application layouts resource on the same server
• See PR discussion for context

[cfisher-scottlogic]

Started working on this whilst I was waiting for #25 to be reviewed. Now that's been merged I'll be focusing on #66, so this can be picked up by someone else.

Branch (WIP): https://github.com/ScottLogic/finos-vuu/tree/VUU81-jsonnode-in-layout